Bitcoin operates a special ledger called blockchain. Thousands of Bitcoin validators around the world are constantly verifying the blocks. Each one who runs this software stores a complete copy of this ledger (full node) and produces new blocks. Each full node validates this ledger by executing the same Bitcoin network rules to make sure no central authority can arbitrarily edit the record to steal Bitcoin or spend the Bitcoin they do not have. Bitcoin network is called public blockchain because anyone can view its transaction history.
A transaction is a transfer of value between two Bitcoin wallets that gets included in the blockchain. Bitcoin wallets keep a secret piece of data called a private key or seed, which is used to sign transactions, providing a mathematical proof that they have come from the owner of the wallet. (instead of an easily forged paper check and handwritten signature) The signature also prevents the transaction from being altered by anybody once it has been issued. All transactions are broadcast to the network and usually begin to be confirmed within 10-20 minutes, through a process called mining.
The name blockchain stems from the fact that it is a collection of all history blocks (or all pages). In another word, the Bitcoin network keeps the complete and unalterable ledger since January 2009.