What is Blockchain?

Blockchain is a distributed, immutable ledger that records transactions across many computers. No single person or company controls it. This is a foundational concept in decentralized application development that professional developers rely on daily. The explanations below are written to be beginner-friendly while covering the depth and nuance that comes from real-world Blockchain/Web3 experience. Take your time with each section and practice the examples

30 min•By Priygop Team•Last updated: Feb 2026

What is Blockchain?

A blockchain is a chain of blocks, where each block contains a list of transactions. Once data is recorded in a block and added to the chain, it cannot be changed without changing all subsequent blocks — which requires agreement from the entire network. This makes blockchain tamper-resistant. Bitcoin (2009) was the first major blockchain. Ethereum (2015) extended it with programmable smart contracts.

Key Properties of Blockchain

Decentralized — No single authority controls it; thousands of computers (nodes) share the ledger
Immutable — Once recorded, data cannot be altered without network consensus
Transparent — All transactions are publicly visible on public blockchains
Trustless — You don't need to trust a bank or company; the code enforces the rules
Secure — Cryptographic hashing links blocks together, making tampering detectable
Permissionless — Anyone can participate in public blockchains like Bitcoin and Ethereum

Traditional Database vs Blockchain

Example

// Traditional Database (centralized)
// - Controlled by one company (e.g., your bank)
// - Can be modified by the company
// - You must trust the company
// - Single point of failure

// Blockchain (decentralized)
// - Controlled by thousands of nodes worldwide
// - Cannot be modified without network consensus
// - You trust the code, not a company
// - No single point of failure

// Example: Sending money
// Traditional: Alice → Bank → Bob (bank can freeze, reverse, or delay)
// Blockchain:  Alice → Network → Bob (no intermediary, irreversible)

const transaction = {
    from: "Alice",
    to: "Bob",
    amount: 1.5,    // BTC or ETH
    timestamp: Date.now(),
    hash: "0x3a4f...",  // Cryptographic fingerprint
};
console.log("Transaction recorded on blockchain:", transaction);

Try It Yourself: Blockchain Concepts

Try It Yourself: Blockchain ConceptsHTML⚠ 3 errors

⚠ Syntax Issues (3)

Line 27: Malformed tag: "< data.length; i++) { hash = ((hash << 5) - hash) + data.charCodeAt(i); hash |= 0; } return "0x" + Math.abs(hash).toString(16).padStart(8, "0").toUpperCase(); } let chain = []; let blockCount = 0; function createBlock(data, prevHash) { const block = { index: blockCount++, timestamp: new Date().toLocaleTimeString(), data: data, previousHash: prevHash, }; block.hash = simpleHash(JSON.stringify(block)); return block; } function renderChain() { const div = document.getElementById("chain"); div.innerHTML = chain.map((b, i) =>"

Tip: Tags must use valid HTML names and be properly formed. Example: <div>, <p class="text">

Line 56: Malformed tag: "< chain.length - 1 ? '<div style="text-align:center;color:#a78bfa;font-size:20px;">"

Tip: Tags must use valid HTML names and be properly formed. Example: <div>, <p class="text">

Line 57: Wrong closing tag: </div> found, expected </script> (line 57)

Tip: Close <script> (opened on line 23) with </script> before closing </div>.

HTML Editor

✕ 3 errorsTab = 2 spaces

<!DOCTYPE html>
<html>
<head>
  <style>
    body { font-family: Arial, sans-serif; padding: 20px; background: #0f0f1a; color: #e0e0e0; }
    h2 { color: #a78bfa; }
    .card { background: #1a1a2e; padding: 20px; border-radius: 10px; margin: 10px 0; border: 1px solid #2d2d4e; }
    .block { background: #16213e; border: 2px solid #a78bfa; border-radius: 8px; padding: 15px; margin: 8px 0; font-family: monospace; font-size: 13px; }
    .hash { color: #34d399; word-break: break-all; }
    .label { color: #a78bfa; font-weight: bold; }
    button { background: #7c3aed; color: white; border: none; padding: 10px 20px; border-radius: 6px; cursor: pointer; margin: 5px; }
    button:hover { background: #6d28d9; }
  </style>
</head>
<body>
  <h2>⛓️ Blockchain Simulator</h2>
  <div class="card">
    <p>See how blocks are linked together in a chain. Each block contains the hash of the previous block.</p>
    <button onclick="addBlock()">➕ Add Block</button>
    <button onclick="resetChain()">🔄 Reset Chain</button>
  </div>
  <div id="chain"></div>
  <script>
    // Simple hash simulation (not real SHA-256)
    function simpleHash(data) {
      let hash = 0;
      for (let i = 0; i < data.length; i++) {
        hash = ((hash << 5) - hash) + data.charCodeAt(i);
        hash |= 0;
      }
      return "0x" + Math.abs(hash).toString(16).padStart(8, "0").toUpperCase();
    }

    let chain = [];
    let blockCount = 0;

    function createBlock(data, prevHash) {
      const block = {
        index: blockCount++,
        timestamp: new Date().toLocaleTimeString(),
        data: data,
        previousHash: prevHash,
      };
      block.hash = simpleHash(JSON.stringify(block));
      return block;
    }

    function renderChain() {
      const div = document.getElementById("chain");
      div.innerHTML = chain.map((b, i) => `
        <div class="block">
          <div><span class="label">Block #${b.index}</span> — ${b.timestamp}</div>
          <div>📦 Data: ${b.data}</div>
          <div>⬅️ Prev Hash: <span class="hash">${b.previousHash}</span></div>
          <div>🔑 Hash: <span class="hash">${b.hash}</span></div>
          ${i < chain.length - 1 ? '<div style="text-align:center;color:#a78bfa;font-size:20px;">↓</div>' : ''}
        </div>
      `).join("");
    }

    const transactions = [
      "Alice → Bob: 1.5 BTC",
      "Bob → Carol: 0.8 ETH",
      "Carol → Dave: 200 USDC",
      "Dave → Eve: 0.05 BTC",
      "Eve → Alice: 50 ETH",
    ];

    function addBlock() {
      const prevHash = chain.length > 0 ? chain[chain.length - 1].hash : "0x00000000";
      const data = transactions[chain.length % transactions.length];
      chain.push(createBlock(data, prevHash));
      renderChain();
    }

    function resetChain() {
      chain = []; blockCount = 0;
      document.getElementById("chain").innerHTML = "<p style="color:#888">Chain cleared. Add blocks above.</p>";
    }

    // Start with genesis block
    addBlock();
  </script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
  <style>
    body { font-family: Arial, sans-serif; padding: 20px; background: #0f0f1a; color: #e0e0e0; }
    h2 { color: #a78bfa; }
    .card { background: #1a1a2e; padding: 20px; border-radius: 10px; margin: 10px 0; border: 1px solid #2d2d4e; }
    .block { background: #16213e; border: 2px solid #a78bfa; border-radius: 8px; padding: 15px; margin: 8px 0; font-family: monospace; font-size: 13px; }
    .hash { color: #34d399; word-break: break-all; }
    .label { color: #a78bfa; font-weight: bold; }
    button { background: #7c3aed; color: white; border: none; padding: 10px 20px; border-radius: 6px; cursor: pointer; margin: 5px; }
    button:hover { background: #6d28d9; }
  </style>
</head>
<body>
  <h2>⛓️ Blockchain Simulator</h2>
  <div class="card">
    <p>See how blocks are linked together in a chain. Each block contains the hash of the previous block.</p>
    <button onclick="addBlock()">➕ Add Block</button>
    <button onclick="resetChain()">🔄 Reset Chain</button>
  </div>
  <div id="chain"></div>
  <script>
    // Simple hash simulation (not real SHA-256)
    function simpleHash(data) {
      let hash = 0;
      for (let i = 0; i < data.length; i++) {
        hash = ((hash << 5) - hash) + data.charCodeAt(i);
        hash |= 0;
      }
      return "0x" + Math.abs(hash).toString(16).padStart(8, "0").toUpperCase();
    }

let chain = [];
    let blockCount = 0;

function createBlock(data, prevHash) {
      const block = {
        index: blockCount++,
        timestamp: new Date().toLocaleTimeString(),
        data: data,
        previousHash: prevHash,
      };
      block.hash = simpleHash(JSON.stringify(block));
      return block;
    }

const transactions = [
      "Alice → Bob: 1.5 BTC",
      "Bob → Carol: 0.8 ETH",
      "Carol → Dave: 200 USDC",
      "Dave → Eve: 0.05 BTC",
      "Eve → Alice: 50 ETH",
    ];

function addBlock() {
      const prevHash = chain.length > 0 ? chain[chain.length - 1].hash : "0x00000000";
      const data = transactions[chain.length % transactions.length];
      chain.push(createBlock(data, prevHash));
      renderChain();
    }

function resetChain() {
      chain = []; blockCount = 0;
      document.getElementById("chain").innerHTML = "<p style='color:#888'>Chain cleared. Add blocks above.</p>";
    }

// Start with genesis block
    addBlock();
  </script>
</body>
</html>

Result

Click ▶ Run to see the result

Edit the code on the left, then click Run

HTML|85 lines|2987 chars|3 errors, 0 warnings

UTF-8

Quick Quiz

What is Blockchain?

30 min•By Priygop Team•Last updated: Feb 2026

What is Blockchain?

Key Properties of Blockchain

Decentralized — No single authority controls it; thousands of computers (nodes) share the ledger

Immutable — Once recorded, data cannot be altered without network consensus

Transparent — All transactions are publicly visible on public blockchains

Trustless — You don't need to trust a bank or company; the code enforces the rules

Secure — Cryptographic hashing links blocks together, making tampering detectable

Permissionless — Anyone can participate in public blockchains like Bitcoin and Ethereum

Traditional Database vs Blockchain

Example

// Traditional Database (centralized)
// - Controlled by one company (e.g., your bank)
// - Can be modified by the company
// - You must trust the company
// - Single point of failure

// Blockchain (decentralized)
// - Controlled by thousands of nodes worldwide
// - Cannot be modified without network consensus
// - You trust the code, not a company
// - No single point of failure

// Example: Sending money
// Traditional: Alice → Bank → Bob (bank can freeze, reverse, or delay)
// Blockchain:  Alice → Network → Bob (no intermediary, irreversible)

const transaction = {
    from: "Alice",
    to: "Bob",
    amount: 1.5,    // BTC or ETH
    timestamp: Date.now(),
    hash: "0x3a4f...",  // Cryptographic fingerprint
};
console.log("Transaction recorded on blockchain:", transaction);

Try It Yourself: Blockchain Concepts

Try It Yourself: Blockchain ConceptsHTML⚠ 3 errors

⚠ Syntax Issues (3)

Tip: Tags must use valid HTML names and be properly formed. Example: <div>, <p class="text">

Line 56: Malformed tag: "< chain.length - 1 ? '<div style="text-align:center;color:#a78bfa;font-size:20px;">"

Tip: Tags must use valid HTML names and be properly formed. Example: <div>, <p class="text">

Line 57: Wrong closing tag: </div> found, expected </script> (line 57)

Tip: Close <script> (opened on line 23) with </script> before closing </div>.

HTML Editor

✕ 3 errorsTab = 2 spaces

<!DOCTYPE html>
<html>
<head>
  <style>
    body { font-family: Arial, sans-serif; padding: 20px; background: #0f0f1a; color: #e0e0e0; }
    h2 { color: #a78bfa; }
    .card { background: #1a1a2e; padding: 20px; border-radius: 10px; margin: 10px 0; border: 1px solid #2d2d4e; }
    .block { background: #16213e; border: 2px solid #a78bfa; border-radius: 8px; padding: 15px; margin: 8px 0; font-family: monospace; font-size: 13px; }
    .hash { color: #34d399; word-break: break-all; }
    .label { color: #a78bfa; font-weight: bold; }
    button { background: #7c3aed; color: white; border: none; padding: 10px 20px; border-radius: 6px; cursor: pointer; margin: 5px; }
    button:hover { background: #6d28d9; }
  </style>
</head>
<body>
  <h2>⛓️ Blockchain Simulator</h2>
  <div class="card">
    <p>See how blocks are linked together in a chain. Each block contains the hash of the previous block.</p>
    <button onclick="addBlock()">➕ Add Block</button>
    <button onclick="resetChain()">🔄 Reset Chain</button>
  </div>
  <div id="chain"></div>
  <script>
    // Simple hash simulation (not real SHA-256)
    function simpleHash(data) {
      let hash = 0;
      for (let i = 0; i < data.length; i++) {
        hash = ((hash << 5) - hash) + data.charCodeAt(i);
        hash |= 0;
      }
      return "0x" + Math.abs(hash).toString(16).padStart(8, "0").toUpperCase();
    }

    let chain = [];
    let blockCount = 0;

    function createBlock(data, prevHash) {
      const block = {
        index: blockCount++,
        timestamp: new Date().toLocaleTimeString(),
        data: data,
        previousHash: prevHash,
      };
      block.hash = simpleHash(JSON.stringify(block));
      return block;
    }

    function renderChain() {
      const div = document.getElementById("chain");
      div.innerHTML = chain.map((b, i) => `
        <div class="block">
          <div><span class="label">Block #${b.index}</span> — ${b.timestamp}</div>
          <div>📦 Data: ${b.data}</div>
          <div>⬅️ Prev Hash: <span class="hash">${b.previousHash}</span></div>
          <div>🔑 Hash: <span class="hash">${b.hash}</span></div>
          ${i < chain.length - 1 ? '<div style="text-align:center;color:#a78bfa;font-size:20px;">↓</div>' : ''}
        </div>
      `).join("");
    }

    const transactions = [
      "Alice → Bob: 1.5 BTC",
      "Bob → Carol: 0.8 ETH",
      "Carol → Dave: 200 USDC",
      "Dave → Eve: 0.05 BTC",
      "Eve → Alice: 50 ETH",
    ];

    function addBlock() {
      const prevHash = chain.length > 0 ? chain[chain.length - 1].hash : "0x00000000";
      const data = transactions[chain.length % transactions.length];
      chain.push(createBlock(data, prevHash));
      renderChain();
    }

    function resetChain() {
      chain = []; blockCount = 0;
      document.getElementById("chain").innerHTML = "<p style="color:#888">Chain cleared. Add blocks above.</p>";
    }

    // Start with genesis block
    addBlock();
  </script>
</body>
</html>

let chain = [];
    let blockCount = 0;

const transactions = [
      "Alice → Bob: 1.5 BTC",
      "Bob → Carol: 0.8 ETH",
      "Carol → Dave: 200 USDC",
      "Dave → Eve: 0.05 BTC",
      "Eve → Alice: 50 ETH",
    ];

function resetChain() {
      chain = []; blockCount = 0;
      document.getElementById("chain").innerHTML = "<p style='color:#888'>Chain cleared. Add blocks above.</p>";
    }

// Start with genesis block
    addBlock();
  </script>
</body>
</html>

Result

Click ▶ Run to see the result

Edit the code on the left, then click Run

HTML|85 lines|2987 chars|3 errors, 0 warnings

UTF-8

What is Blockchain?