What is a block chain?

A block chain is in the most basic terms a ledger that is used for recording transactions in a network.  Some of the most popular block chains are etheryum, and bitcoin.  The idea when you purchase cryptocurrency is you make a new transaction in the blockchain and your transaction gets a copy of every single transaction prior to yours from the general ledger.  Everything is trackable this way and every token can be accounted for.  Nothing will ever get lost.