Each blockchain block contains:

1. Data: depends on the type of chain. Bitcoin stores the sender, receiver, and amount.
2. Hash: a fingerprint that identifies the block. If the content changes the hash will not match and the interference will be detected. 
3. Hash of the previous block: This is what creates a chain because each block point to its subsequent one. Hashes are not generated en mass because they require a lot of computing power, through a method called proof-of-work.