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 20 witnesses (by vote), plus one randomly-selected standby 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, its a Witnesses :
@jesta , @roelandp , @gtg , @good-karma , @timcliff , @curie , @blocktrades , @ausbitbank , @someguy123 , @aggroed , @smooth.witness , @lukestokes.mhth , @anyx , @clayop , @pharesim , @thecryptodrive , @furion , @pfunk , @xeldal, @netuoso , @liondani , @riverhead , @jerrybanfield , @wackou , @bhuz , @busy.witness , @utopian-io , @followbtcnews , @cervantes , @drakos , @teamsteem , @klye , @abit , @yabapmatt , @aizensou , @charlieshrem , @ihashfury , @reggaemuffin , @blockbrothers ,   @joseph , @steempty , @prc , @fyrst-witness , @steemed , @rival , @ocd-witness , @patrice , @delegate.lafona , @therealwolf , @nextgencrypto , @themarkymark , @liberosist , @ats-witness , @neoxian , @steemgigs , @dragosroua , @steemychicken1 ,         @krnel , @boatymcboatface , @arcange , @chitty , @chainsquad.com , @adsactly-witness 	, @privex , @au1nethyb1 , @blueorgy , @blockchained , @asbear , @sircork , @viva.witness , @b0y2k , @gmuxx , @kushed , @felixxx , @sc-steemit , @bue , @masteryoda , @jatinhota , @demotruk , @suggeelson , @libertyranger , @yehey , @block-buster , @swelker101 , @anarcho-andrei , @jacor-witness

Link Source : https://steemd.com/witnesses