Bitcoin Forum

What is a blockchain? In simpleton terms..

It's a big file which consists of many smaller parts (blocks - which you could think of as "pages").

Every computer that runs the Satoshi client has it's own copy of this big file and it is fundamental to the way that Bitcoin can work as a decentralised P2P system.

The blocks contain all the Bitcoin transactions - so actually the "blockchain" is a "shared ledger" between all the Satoshi clients.

From the wiki: A block chain is a transaction database shared by all nodes participating in a system based on the Bitcoin protocol. A full copy of a currency's block chain contains every transaction ever executed in the currency. With this information, one can find out how much value belonged to each address at any point in history.
So esentially, it's the database that holds all transactions. That's all that you need to know.

In simpleton terms?  A "blockchain" is a chain of blocks.

To get a bit more complex, it is a crypto-currency transaction ledger.

The "blocks" are collections of transactions that are added to the database together all at the same time.  Each block of transactions has a header (some information before the transactions are listed) with important details about the data in the block, as well as information that qualifies the block to be added to the database.  One of the pieces of information in the header is an identifier that uniquely identifies the previous block added.  In this way there is a "chain" of blocks each identifying the previous block all the way back to the first block.