A blockchain is a ledger of transactions, which (for all intents and purposes) can’t be altered or reversed.
Think of a Blockchain as a database for transactions that doesn’t require trusted administrators.
Bitcoin is built on top of a blockchain, in which the properties of encryption give people power over their own money. The Bitcoin network was designed to allow people with no particular financial expertise to have equal access to the network and to trade and spend virtual coins (Bitcoin) as currency.
A blockchain also helps to virtually eliminate fraud by distributing records of financial transactions across a multitude of different computer systems within its network so that no one person alone has a record of the transaction.