Steemd has a nice summary here - at the bottom of the page as well. Here is the quote:
On the Steem blockchain, witnesses serve a role of validating signatures and timestamping transactions by including them in blocks. A block is any group of transactions (posts, votes, transfers, etc) which update the state of the database. Each time a witness produces a block, they are paid for their service. If a witness fails to produce a block, then they are not paid, and may even be voted out.
Every round of block production begins with the shuffling of 21 witnesses: the top 19 witnesses (by vote), plus one randomly-selected standby witness, and one miner witness. Each is given a turn to produce a single block at a fixed rate of one block every 3 seconds. If a witness does not produce a block in their time slot, then that time slot is skipped, and the next witness produces the next block.