A block is a group of Bitcoin transactions. It is also used to describe a unit of Bitcoin mining. A transaction becomes confirmed by being included in a block. Conversely, transactions that do not belong to any confirmed blocks are considered to be unconfirmed.
Through the mining process, new blocks containing transactions are added to the blockchain approximately every 10 minutes. When a block is mined, bitcoins are issued to the miner to serve as compensation for their work.
Bitcoin began with a starting block called "Genesis." As of October 2015, there are 370,000 blocks. One block is added every 10 minutes.