What is a Blockchain? How It Works Explained Simply

How blocks and chains work

Let's walk through what actually happens when you send crypto to someone.

Say you send 0.5 ETH to a friend. Your wallet creates a transaction and signs it with your private key (that's your digital signature proving you authorized it). This transaction gets broadcast to the Ethereum network.

Thousands of nodes receive your transaction. They check that you actually have 0.5 ETH and that your signature is valid. If everything checks out, your transaction goes into a waiting room called the mempool with all the other pending transactions.

Every few seconds (on Ethereum it's about 12 seconds), a validator gets chosen to create the next block. They pull transactions from the mempool, bundle them together, and package them into a block. A block typically contains hundreds or thousands of transactions.

Each block contains three key pieces of information: the transaction data, a timestamp, and a reference to the previous block (its hash). That reference is what creates the "chain" part. Block 500 contains a fingerprint of block 499, which contains a fingerprint of block 498, and so on. It's like a chain of trust going all the way back to the beginning.

Once the block is created and verified by other nodes, it gets added to the chain permanently. Your friend's wallet now shows 0.5 ETH more, and your wallet shows 0.5 ETH less. The transaction is final. Nobody can reverse it, edit it, or pretend it didn't happen.

Hashing: the glue that holds it together

If there's one concept you need to understand about blockchains, it's hashing. Don't worry, it's simpler than it sounds.

A hash is a digital fingerprint. You take any piece of data, whether it's a sentence, a photo, or a whole block of transactions, run it through a hash function, and you get a fixed-length string of characters. The same input always produces the same output. But change even one letter and the output changes completely.

For example, the SHA-256 hash of "Hello" is 185f8db32271fe25f561a6fc938b2e264306ec304eda518007d1764826381969. Change it to "hello" (lowercase h) and you get a completely different hash: 2cf24dba5fb0a30e26e83b2ac5b9e29e1b161e5c1fa7425e73043362938b9824. Completely different output from one tiny change.

Here's why this matters for blockchains. Each block's hash is calculated from its contents plus the hash of the previous block. If someone tried to change a transaction in block 100, the hash of block 100 would change. But block 101 contains block 100's old hash, so now there's a mismatch. You'd have to recalculate block 101, which changes its hash, which breaks block 102, and so on. You'd need to redo every single block after the one you changed.

On a network with thousands of nodes all holding their own copy, tampering with one copy doesn't help you. The other nodes would reject your modified version because it doesn't match theirs. You'd need to change the data on more than half the nodes simultaneously, which is practically impossible on major blockchains. This is the "immutability" that people talk about.

Consensus mechanisms explained

When there's no central authority deciding what's true, how do thousands of computers agree on the state of the blockchain? That's the job of consensus mechanisms.

Proof of Work (PoW): This is what Bitcoin uses. Miners compete to solve complex math puzzles. The first one to find the answer gets to add the next block and earns a reward. Solving the puzzle requires enormous amounts of electricity and computing power, which makes cheating extremely expensive. The downside? PoW uses as much energy as some small countries. Bitcoin's network alone consumes more electricity than Argentina.

Proof of Stake (PoS): This is what Ethereum and most modern blockchains use. Instead of solving puzzles, validators lock up (or "stake") their tokens as collateral. The network randomly picks validators to create blocks based on how much they've staked. If a validator tries to cheat, they lose their staked tokens. It uses a fraction of the energy of PoW while still being secure.

Delegated Proof of Stake (DPoS): Used by chains like Solana and EOS. Token holders vote for a smaller set of validators to process transactions on their behalf. It's faster but more centralized, with fewer validators controlling the network.

There are other variations too, like Proof of History (Solana), Proof of Authority (used in private blockchains), and Byzantine Fault Tolerance systems. Each one makes different tradeoffs between speed, security, and decentralization. This is sometimes called the "blockchain trilemma" because it's really hard to maximize all three.

Types of blockchains

Not all blockchains are created equal. They fall into a few categories:

Public blockchains are open to everyone. Anyone can run a node, send transactions, or become a validator. Bitcoin and Ethereum are public blockchains. They're the most decentralized and transparent, but they can be slower and more expensive because every node needs to process every transaction.

Private blockchains are controlled by a single organization. Only approved participants can view or add data. Companies like JPMorgan and Walmart use private blockchains for supply chain tracking and internal processes. They're faster but not really decentralized, which kind of defeats the purpose in the eyes of crypto purists.

Consortium blockchains are somewhere in between. A group of organizations jointly controls the network. Think of several banks running a shared blockchain for interbank settlement. It's more decentralized than a private blockchain but not open to the public.

Layer 1 vs Layer 2: The main blockchain (like Ethereum) is called Layer 1. Layer 2 networks are built on top of Layer 1 to handle transactions faster and cheaper, then batch the results back to the main chain for security. Arbitrum, Optimism, and Base are popular Ethereum L2s.

In the crypto world, when people say "blockchain" they almost always mean public blockchains. That's where all the interesting stuff, like DeFi, NFTs, and DAOs, lives.

Real-world blockchain examples

Bitcoin: The first and most famous blockchain. Launched in 2009, it's a peer-to-peer payment network and store of value. The Bitcoin blockchain processes about 7 transactions per second and has never been hacked in its 17 years of existence. It's simple by design: it does one thing (transfer value) and does it well.

Ethereum: Launched in 2015, Ethereum added smart contracts to the blockchain concept. This turned a simple ledger into a programmable computer. Developers can build applications that run automatically and can't be censored. Ethereum hosts the majority of DeFi protocols, NFT marketplaces, and decentralized apps.

Solana: A newer blockchain designed for speed. Solana processes thousands of transactions per second with fees under a penny. It's popular for trading, memecoins, and apps that need fast finality. The tradeoff is fewer validators and some historical reliability issues.

Supply chain tracking: Walmart uses a blockchain to track food from farm to shelf. When there's a contamination scare, they can trace the source in seconds instead of days. De Beers uses blockchain to verify that diamonds aren't conflict diamonds. These are real, boring, practical uses of the technology.

Stablecoins: USDC and USDT are dollar-pegged tokens that move on blockchains. They process billions in transfers daily, often faster and cheaper than traditional wire transfers. A $100 million USDC transfer costs a few dollars in fees and settles in minutes.

Pros and cons of blockchain technology

The scaling problem is the biggest challenge. Bitcoin handles about 7 transactions per second. Ethereum does around 30. Visa does 65,000. That gap is why Layer 2 solutions and newer blockchains like Solana exist. They're all trying different approaches to make blockchains fast enough for mainstream use.

The environmental concern has gotten better. Ethereum cut its energy usage by 99.95% when it switched from proof of work to proof of stake in 2022. Bitcoin still uses PoW, though, and that's unlikely to change.