Honestly I was actually expecting you to make a proper research on this question before putting it up, haven't known the purpose of this forum.Nevertheless..

Bitcoins is a form of virtual currency- meaning, if you have bitcoins you do not physically purchase goods by handing notes or tokens to the seller. Bitcoins are used for electronic purchases and transfers. You can use bitcoins to pay friends, merchants, etc. Every single purchase is immediately logged digitally (on computers) on a transaction log that tracks the time of purchase and who owns how many bitcoins. Think of this transaction log as an audit trail: it contains every single piece of information of every bitcoin transaction. This digital transaction log is called 'blockchain'.

The blockchain records every single transaction - of present and past - and the ownership of every single bitcoin in circulation.

Though there are still more information about Bitcoin.