The blockchain is a shared digital ledger which holds a record of all bitcoin transactions. Each cryptocurrency transaction is grouped together into 'blocks' by miners, who compete to add new blocks to the blockchain by solving complex mathematical problems. In return, miners are rewarded with newly created bitcoins.
The transactions are then cryptographically secured before they are added to the existing blockchain. Each 'node' or computer connected to the network automatically downloads a copy of the blockchain which allows everyone to track transactions without the need for central record keeping.
The blockchain is accessible to everybody at any time. The shared record can't be changed without the agreement of the rest of the network.