Consensus is the process by which a decentralised network of computers agrees on a single version of the truth. In a blockchain, the “truth” is the order and contents of transactions — who paid whom, in what order, and what the current balance of every address is. Without consensus, a blockchain is just a bunch of computers maintaining different versions of a ledger and arguing about which one is correct. With consensus, they all agree.
The problem is harder than it sounds. If all the computers in the network are trusted and reachable, consensus is trivial — they just vote. The interesting case is when some of them are adversarial (trying to double-spend, censor, or rewrite history), or offline, or on the wrong side of a network partition, and the network has to produce a consistent answer anyway. This is the problem the Byzantine Generals Problem describes, and it is what every blockchain consensus mechanism is trying to solve.
The Two Big Families
In crypto today, consensus comes in two main flavours. Proof-of-work is what Bitcoin uses. Nodes compete to solve a computationally expensive puzzle, and the first one to solve it gets to propose the next block. Attacking the network requires buying enough computing power to outpace everyone else, which is expensive and visible.
Proof-of-stake is what Ethereum, Solana, Cardano, Avalanche and most newer chains use. Instead of competing via computation, validators lock up coins as collateral, and the protocol rotates the right to propose blocks among them weighted by how much they have staked. Attacking the network requires buying or borrowing a very large fraction of the total stake, and the protocol can slash (destroy) the stake of anyone who misbehaves.
Both families have subvariants. Nakamoto-style PoW (Bitcoin, Litecoin, Dogecoin) versus hybrid PoW/BFT approaches. Gasper (Ethereum’s specific PoS design) versus Tendermint (used by Cosmos) versus Solana’s proof-of-history-plus-tower-BFT. These differ in how they handle finality, how they penalise bad behaviour, and how they trade off safety against liveness under network stress. The differences matter if you are building a chain or writing about one. For most users they are implementation details.
What Consensus Does Not Do
One common misunderstanding: consensus does not verify that transactions are correct in any meaningful sense. It verifies that they follow the rules of the protocol — signatures are valid, the sender has the balance, the transaction format is well-formed — and then orders them and commits them. Whether the transaction is a legitimate payment, a scam, a mistake, or a brilliant arbitrage opportunity is not something the protocol has any opinion on. That is left to the people sending the transactions.
This is the reason the phrase “code is law” exists in crypto. If you sign a transaction and it executes, the chain considers it final. Whether you meant to do it is your problem, not the network’s.