Bitcoin 101 explained why Bitcoin matters. This guide is about how it actually runs. There's no Bitcoin company, no head office, and no master server, just thousands of independent computers around the world, following the same rules and agreeing on the same ledger. Here's how that works.
Nodes: The Backbone
A node is simply a computer running Bitcoin software. A full node does three jobs that make the whole system tick:
- Stores the blockchain: its own complete copy of every transaction ever made. A brand-new node performs an initial block download, re-verifying the entire history from the very first (genesis) block rather than trusting anyone's summary.
- Validates everything: it checks every transaction and block against Bitcoin's consensus rules (valid signatures, no double-spends, the correct block reward, the 21-million cap, and more). To catch double-spends instantly, it keeps a UTXO set, a running index of every coin currently available to spend.
- Relays: it passes valid data on to other nodes.
Crucially, every node is equal, and each one verifies for itself. Nobody is in charge. This is where the phrase rules without rulers becomes concrete: the rules are enforced by everyone, not by anyone. (Short on disk? A pruned node validates the full history but then discards old block data, running in just a few gigabytes.)
A Peer-to-Peer Gossip Network
Nodes don't connect to a central hub. Each one finds a handful of peers (bootstrapping from a few known starting points) and information spreads by gossip: you tell your peers, they tell their peers, and within a few seconds a message has flooded across the entire planet. There's no broadcaster to shut down and no switchboard to tap. The network has no centre. Many people run their node over Tor for added privacy.
The Life of a Transaction
When you send bitcoin, your wallet broadcasts the transaction to a few nodes. Each node that receives it validates it (are the signatures valid? do the inputs exist in the UTXO set and remain unspent?) and, if it checks out, holds it in its mempool (the pool of pending transactions) and relays it onward. Nodes apply a relay policy too: a transaction has to clear a minimum feerate and look standard, or peers simply won't pass it along. If a transaction is outright invalid, honest nodes drop it. The network polices itself at every hop.
Mining: Turning Energy Into Order
Pending transactions need to be put in a permanent order that everyone agrees on. That's the job of miners. A miner gathers transactions from the mempool into a candidate block and summarizes it in an 80-byte block header containing: a version, the previous block's hash (this is the literal "chain" link), a Merkle root (a single fingerprint that commits to every transaction in the block, built by hashing them together in a Merkle tree), a timestamp, the target, and a counter called the nonce. Then it competes to find a valid proof-of-work:
- It runs the header through the SHA-256 hash function (twice) over and over, changing the nonce each time, searching for a result below a network-wide target. The target is stored compactly in a header field called bits.
- There's no shortcut. It's pure trial and error, and the work is done by purpose-built chips (ASICs), usually combined into mining pools. The global guessing rate (the hashrate) runs in the quintillions of guesses per second (exahashes).
- On average a miner wins this lottery about every 10 minutes. Every 2,016 blocks (~two weeks) the difficulty automatically re-tunes to hold that pace as mining power comes and goes.
The reward lives in the block's first transaction, the coinbase, which pays the winner the block subsidy plus all the fees they included. The subsidy started at 50 BTC and is cut in half every 210,000 blocks (~four years) in the event we call the halving. It's 3.125 BTC today and trends toward zero, after which miners are paid by fees alone. That very real energy cost is the point: it's what makes rewriting history prohibitively expensive.
Blocks and Validation
When a miner finds a valid block, it gossips out across the network just like a transaction. Then every node independently re-checks it: is the proof-of-work actually below the target? is every signature and script valid against the UTXO set? are there no double-spends? is the block within the ~4 million weight-unit size limit? does the coinbase claim the correct reward and no more? (And those freshly minted coins can't be spent for 100 blocks, a rule called coinbase maturity.) Only if everything passes does a node add the block and start building on top.
This is the part people miss: miners propose, but nodes decide. A miner who tries to sneak in a fake transaction or print extra coins simply has their block rejected by every honest node, forfeiting all that electricity for nothing.
Consensus: The Most-Work Chain
With thousands of independent nodes, how does everyone agree on one history? They all follow a single tie-breaking rule: the valid chain with the most cumulative proof-of-work wins. That means the most total work (the sum of all that difficulty), not simply the most blocks. Each block's header points back to the one before it, forming the chain; the "heaviest" chain is the truth.
To rewrite recent history you'd need a majority of the world's hashrate, a so-called 51% attack. And even then you could only reorder or censor recent transactions. You still couldn't steal coins, forge a signature, or mint extra bitcoin, because every node would reject those outright. Proof-of-work also solves a subtler problem, Sybil resistance: you can't gain influence by spinning up thousands of fake nodes, because influence is bought with real energy, not identities.
When Two Blocks Collide: Reorgs
Occasionally two miners find a block at almost the same moment, and the network briefly splits: some nodes see one, some see the other. The tie breaks at the next block: nodes switch to whichever branch now has more work, and the transactions from the abandoned (orphan) block return to the mempool to be mined again. This is a reorganization, and it's exactly why waiting a confirmation or two adds safety. See Using Bitcoin as Money.
Changing the Rules: Forks
Because nodes enforce the rules, changing them is a social process, not a command. A soft fork tightens the rules in a backward-compatible way (older nodes still accept the new blocks), and it's how careful upgrades like SegWit and Taproot shipped. A hard fork loosens them and requires everyone to upgrade or split off. Either way, no developer or miner can force a change: it takes hold only if users choose to run the software. The users hold the final say.
Why It Matters: No Gatekeepers
Anyone with a computer and an internet connection can join. No application, no permission. That means no one can stop your transaction from spreading, and no one can quietly change the rules. To censor or rewrite Bitcoin, an attacker would need to out-compute the whole honest network at once, which is astronomically expensive. The result is censorship resistance: money that works the same for everyone, everywhere.
Run Your Own Node
You don't have to take anyone's word for any of this. A light wallet (SPV) trusts other people's summaries of the chain; a full node trusts no one. Running your own is the purest form of don't trust, verify: it enforces every rule for you, checks every payment you receive against the real consensus rules, frees you from third parties, and adds one more independent guardian to the network. It's also more accessible than it sounds: modest hardware and a download is all it takes (a pruned node keeps disk use tiny), and you can point a wallet like Sparrow straight at it.
Keep going. Put it into practice with Using Bitcoin as Money, pick a wallet on Choosing a Wallet, or bring your questions to our next meetup.