what's a Steemit witness?
They are voted on by owners of Steem accounts, using a steem-power weighted system, so they are generally some of the most trusted members of the community, as well as being tech savvy. They are expected to keep a node running every second of every day.
Blocks are produced in rounds, 63 seconds per round and 21 blocks per round. Every round, the top 19 witness accounts are delegated to produce a block, 1 block is produced by a backup witness, and 1 by a miner.