A 'Master of Ceremonies'. One who raps to inform the public, trying to uplift people rather than degrade. Of course, emcees have to have the ability to 'control' the crowd, and exhibit good showmanship. Emcees use thoughtful lyrics over a beat. Most emcees need to be able to freestyle, to rap whatever comes to mind in order to show real skill.
Some skilled emcees are MC Supernatural, Mos Def, KRS-One, Nas, Tupac, B.I.G.