As you might imagine, the Beastmaster commands beasts and the power of nature. Its main role is that of summoner and crowd control, but with heavy mail armor and two-handed weapons at its disposal, the class can also do a fair bit of tanking.
Special moves include Law of the Jungle, a stealth ability that increases critical hit damage; Thunder-crack, a spell that summons a powerful tropical storm; and Spirit Creature, an area-of-effect spell that causes damage to a wide area.
Be sure to check out the official class announcement for more info and screenshots of this new class.
[Thanks to Novas for the tip!]