Data is permanently recorded in the Bitcoin network through files called blocks. A block is a record of some or all of the most recent Bitcoin transactions that have not yet been recorded in any prior blocks. They could be thought of like the individual pages of a city recorder's recordbook (where changes to title to real estate are recorded) or a stock transaction ledger. In all but a few exceptional cases, new blocks are added to the end of the record (known in Bitcoin as the block chain), and once written, are never changed or removed. Each block memorializes what took place immediately before it was created.
