Blockchain is a digitized, decentralized, and the public ledger of all cryptocurrency transactions. It is constantly growing as "completed" blocks; usually the most recent transactions are put down and added to it in a chronological order. It gives authorizations to market participants to monitor digital currency transactions without central record keeping.  Every computer connected to the network gets a copy of the blockchain, and by default it downloads the copy automatically.

Blockchain is not bitcoin. Blockchain is just the main technological innovation of bitcoin, same as with other cryptocurrencies.