Blockchains

The first blockchain, Bitcoin, was created in 2008. Bitcoin consists of a chain of blocks where each block is cryptographically linked to the previous one. A block contains transactions. A transaction has one or more inputs and one or more outputs. An output locks a certain amount of bitcoin so it can only be spent by someone with the correct cryptographic key. An output of a transaction can be used as an input to a following transaction. To get the balance of an address you have to work out how much bitcoin is stored in all the unspent transaction outputs (UTXOs) that the key which controls the address can unlock.

Next came Ethereum. This is also a chain of blocks containing transactions. From this point onwards though it differs from Bitcoin. Ethereum uses accounts to keep track of balances rather than UTXOs. Each account has a balance that is either debited or credited through a transaction. An account can belong to a person or a smart contract - a program that runs on the blockchain. This opens up a whole new world of possibilities that Bitcoin doesn't have.

Newer blockchains take the successful parts of Bitcoin or Ethereum and try to solve issues that they see in them like scaling and consensus problems. Most are build with running on-chain programs in mind.