Blockchain is a specific technology, but there are many forms and varieties. For instance, Bitcoin and Ethereum are proof-of-work blockchains. Ethereum has smart contracts, and many blockchains allow custom tokens. Blockchains can be differentiated by their consensus algorithm (PoS, PoW, and others)—covered in Chapter 7, Achieving Consensus, and their feature set, such as the ability to run smart contracts and how those smart contracts operate in practice. All of these variations have a common concept: the block. The most basic unit of a blockchain is the block. The simplest way of thinking of a block is to imagine a basic spreadsheet. In it, you might see entries such as this:
| Account | Change | New Balance | Old Balance | Operation |
| Acct-9234222 | −$2,000 | $5,000 | $7,000 | Send funds to account-12345678 |
| Acct-12345678 | $2,000 | $2,000 | 0 | Receive funds from account-9234222... |