What is the Block Chain?

Block Chain– A public ledger. The record of all bitcoin transactions, ever. The block chain is used to reference what address owns X ammount of bitcoin. If I send bitcoin address “ABC” 5 bitcoins and you send address “ABC” 0.1 bitcoin then the when you type “ABC” into a block chain explorer you will be able to see that “ABC” has 1.1 bitcoin. You can also see how much that address has sent or recived.
You can take a closer look at BlockChain.info