According to the announcement post, the Cleric is a battle priest who's big on doing nice things for good folks while smashing bad folks with blunt instruments. The Cleric uses divine spells to heal friends and harm foes backed up with his or her trusty warhammer or flail and a lot of heavy armor. It looks as though players will have an option to eschew this mode and spec as a lightly armored spellcaster or an astral Paladin.
One of the most interesting aspect of the Cleric is his or her choice in deity. That choice won't be merely for roleplaying purposes but will influence which spells and abilities will be granted to the Cleric.