A blockchain is a digital copy of a public ledger that contains the transaction records of every bitcoin payment that has ever been made. It is decentralised as no one entity has complete control over the ledger and it can also be easily downloaded. New records of completed transactions are constantly being added to the block in chronological order and the movement of bitcoins can easily be traced from one node to another.
Blockchains use a distributed ledger technology, an accounting method for bitcoins but it has been applied to a number of other commercial applications.

A block is a part of the blockchain which records some part of the bitcoin transactions. Each block is dependent on the other and as such are stacked in order to create a flow. A current block must be completed before another can be started. Once completed, the block goes into the blockchain permanently. Every block contains a hash or "index" for the previous in order to maintain the linear flow. Blocks are added through cryptography and as such cannot be tampered with. They can be copied but not distributed.
A block is completed approximately every 10 minutes by bitcoin miners and they receive bitcoins as rewards for completing that block. Monitoring the blockchain and ensuring transactions are valid are part of the ways they keep the network secure from fraud.

To view the blockchain, you can visit https://blockchain.info/ .