This article contains the best explanation of what a blockchain is that I have ever seen: A blockchain is a specific data structure: a linear transaction log,

This article contains the best explanation of what a blockchain is that I have ever seen: A blockchain is a specific data structure: a linear transaction log, typically replicated by computers whose owners (called miners) are rewarded for logging new transactions. There are two things that are cool about this particular data structure. One is that a change in any block invalidates every block after it, which means that you can’t tamper with historical transactions. The second is that you only get rewarded if you’re working on the same chain as everyone else, so each participant has an incentive to go with the consensus. The end result is a shared definitive historical record. And, what’s more, because consensus is formed by each person acting in their own interest, adding a false transaction or working from a different history just means you’re not getting paid and everyone else is. Following the rules is mathematically enforced—no government or police force need come in and tell you the transaction you’ve logged is false (or extort bribes or bully the participants).