A block is a group of transactions bundled together and added to a blockchain as a single unit. Rather than recording each transaction one by one, networks collect many into a block, confirm them together, and then attach the block to the chain of blocks that came before it.
Each block carries a reference to the one before it, a kind of fingerprint of the previous block contents. This linking is what forms the chain and what makes tampering difficult. Changing a past block would alter its fingerprint, breaking every link that follows, so the further back a block sits, the harder it is to change.
Blocks are added at regular intervals that vary by network. Bitcoin aims for a new block roughly every ten minutes, while other chains are faster. Each new block extends the shared history that all participants agree on, which is what keeps the record consistent across the network.
