A block is a collection of transactions that occur on the Bitcoin network. These blocks are linked together chronologically to form a blockchain. Most Bitcoin blocks include around 2700 transactions and can be up to 4MB in size.
A block can only be added to the blockchain if it has a hash that satisfies Bitcoin’s Proof-of-Work requirement and includes the previous block’s hash. The inclusion of the previous block’s hash in a block ensures that no block can be altered without invalidating subsequent blocks. This trait is due to the nature of hash functions, which are deterministic and random.
For example, if a transaction in Block #400 is altered, the hash of Block #400 will change, invalidating the Proof-of-Work for Block #400, but also making Block #401 invalid, because Block #401’s “previous hash” parameter no longer matches the hash of Block #400. This feature ensures that once a block is added to the blockchain neither it nor any transactions included within it can be altered.