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What Is a Blockchain?

A blockchain is a decentralized, tamper-proof ledger shared across thousands of computers. Learn how it works, who validates transactions, and why it matters.

What Is a Blockchain?

A blockchain is a shared ledger maintained by thousands of independent computers — no bank, no government, no single point of failure.

TL;DR

  • A blockchain eliminates trusted intermediaries (banks, PayPal) by using a decentralized network of computers
  • Every transaction is verified by thousands of nodes simultaneously, then permanently recorded
  • Bitcoin uses Proof of Work; Ethereum uses Proof of Stake since "The Merge" in 2022
  • Smart contracts are self-executing programs on the blockchain — the foundation of DeFi, NFTs, and DEXs
  • "Not your keys, not your coins" — self-custody is the only true ownership

The Problem Blockchain Solves

When you send money to someone today, you always need a middleman — a bank, PayPal, Revolut. That middleman confirms you have the funds and authorizes the transfer.

The problem: you have to trust them. And that trust has a cost — fees, delays, geographic restrictions, censorship risk.

Blockchain eliminates this trusted third party. Instead of relying on one institution, a decentralized network of thousands of computers holds the same ledger and verifies every transaction simultaneously. No intermediary fees. No censorship. No single point of failure.

Decentralization and Nodes

Instead of one central server, a blockchain uses thousands of computers spread across the world, each holding a complete copy of the ledger. This is a decentralized network — and each participating computer is a node.

When you send ETH to someone:

  1. Your transaction is broadcast to the entire network
  2. Nodes verify it simultaneously — do you have the funds? Is the signature valid?
  3. Once validated, it gets bundled into a block
  4. The block is appended to the chain → blockchain

Each block is cryptographically linked to the one before it. Altering any single block would break every block that follows — making tampering practically impossible.

This tamper-proof public ledger is what makes trading, DeFi, smart contracts, and NFTs possible.

Proof of Work vs Proof of Stake

For a block to be added to the chain, the network needs to agree on who gets to write it. This is the consensus mechanism.

⛏️ Proof of Work (PoW) — Bitcoin

Miners compete to solve a mathematical puzzle based on the block's hash — a unique, irreversible digital fingerprint:

"Hello" → a7f3c9...
"hello" → 2k91md...

Miners test billions of combinations per second. The first to find the correct hash validates the block and earns BTC. Finding the answer requires billions of attempts — verifying it takes a single calculation. That asymmetry is what makes PoW secure.

Drawback: enormous energy consumption.

🔒 Proof of Stake (PoS) — Ethereum

Instead of burning energy, validators lock exactly 32 ETH as collateral in a smart contract.

  • Validate honestly ✅ → earn ETH rewards (~3–4% APY)
  • Try to cheat ❌ → get slashed — a portion of your stake is burned

The economic incentive replaces brute-force security. PoS doesn't rely on "trust" — it relies on the fact that cheating costs you real money.

Ethereum switched from PoW to PoS in September 2022: "The Merge" → energy consumption down 99%.

Gas Fees

Every transaction costs gas fees — the payment to validators or miners for processing your transaction.

The amount depends on:

  • Complexity — sending ETH = low gas / interacting with a smart contract = more gas
  • Congestion — high network traffic = higher fees (like Uber surge pricing)

On Ethereum mainnet: $10–50 per transaction. On Base (L2): a few cents. That's exactly why L2s exist — covered in Lesson 2.

Smart Contracts

A smart contract is a program deployed on the blockchain that executes automatically based on predefined rules — no human intermediary needed.

Example:

"If this address sends 1 ETH before April 1st, release 1,000 tokens. Otherwise, refund."

Once deployed, no one can stop it, modify it, or cheat it. It is the foundational building block of the entire crypto ecosystem.

What smart contracts replace: lawyers, notaries, banks, escrow agents.

What they enable:

  • DeFi — smart contracts that perform financial operations
  • NFTs — smart contracts that prove and transfer ownership
  • DEXs (Uniswap...) — smart contracts that execute trades automatically

Coins vs Tokens

Coins

A coin is the native currency of a blockchain network:

  • BTC on Bitcoin
  • ETH on Ethereum
  • SOL on Solana

Coins pay gas fees and secure the network. They exist at the protocol level.

Tokens

A token is created by a smart contract on top of an existing network:

  • USDC — dollar-pegged stablecoin
  • UNI — Uniswap governance token
  • Memecoins

Anyone can create a token in minutes. That's why 99% of tokens eventually go to zero — there is no technical barrier to creating them.

Immutability — A Double-Edged Sword

Once a transaction is written to the blockchain, it is permanent.

Advantages:

  • No one can erase your transaction history
  • No one can seize or freeze your funds
  • Everything is publicly verifiable

Consequences:

  • Errors are final — wrong address → funds lost, no customer support
  • Full responsibility — losing your seed phrase (12 words) means permanent loss of access
  • Pseudonymous, not anonymous — every address's full history is public

"Not Your Keys, Not Your Coins"

If someone else holds your private keys — a centralized exchange like Binance or Coinbase — those are technically their coins. You hold a claim on the exchange, not the assets.

If the exchange gets hacked, goes bankrupt, or freezes withdrawals, you lose everything. FTX collapsed in 2022 — millions of people lost their funds overnight despite believing they were "safely" stored. The only real ownership is self-custody.

Blockchain Basics