From: https://en.wikipedia.org/wiki/Bitcoin#Block_chain

"Integral to Bitcoin is a sequential record of all transactions known as the block chain. There is only one block chain, and it records bitcoin ownership both present and at all points in the past. By keeping a record of all transactions, the block chain mitigates double-spending, a core design challenge of digital currency. Approximately every ten minutes a new bundle of transactions, called a block, is added to the block chain."

If you need a more in depth explanation, which also covers how this relates to a famous problem in distributed systems (the Byzantine Generals Problem), see: http://expectedpayoff.com/blog/2013/03/22/bitcoin-and-the-by...