A STEEM witness is a person whom operates a witness server (which produces blocks), and publishes a price feed of STEEM/USD to the network.

Basically they also update the servers to the new code when a Hard Fork happens. Everyone can vote on the witnesses just like you vote on a post by going to https://steemit.com/~witnesses

You will see the witnesses announce updates once in awhile and also campaign for your vote to be a witness. You can also become a witness and campaign for a top spot!

Eventually I will run a witness but I need to gain more credibility on the platform and I want to do some programing for the platform before I roll out a witness. If you don't get in the top 40 or so slots then it won't be profitable to run a witness. If you get in the top 19 spots it can be very profitable.