Blockchain can’t be left outside any discussion that has to do with “bitcoin protocol”. The blockchain being a public ledger keeps record of all bitcoin transactions that has ever been executed since it’s invention.
It is seen as the main technological innovation of Bitcoin, since it stands as proof of all the transactions on the network. A block is the ‘current’ part of a blockchain which records some or all of the recent transactions, and once completed goes into the blockchain as permanent database. With blockchain, transactions are carried out without a middleman, thus making traditional banking system obsolete.
The blocks are added to the blockchain in a linear, chronological order. Every node computer linked to the Bitcoin network utilizing a client that is in charge of validating and relaying transactions gets a copy of the blockchain, which gets downloaded automatically upon becoming a member of the Bitcoin community.
The blockchain has whole information concerning the addresses and their balances right from the genesis block to probably the most not too long ago completed block.