Both are card games. Wizard is a pretty simple trick taking game, Citadels is a bit more complex (but still easy). Players build cities with district cards while selecting secret hidden roles each round.
Here's my review of Wizard, should give a good overview: http://www.boardgamegeek.com/thread/298041
Not quite 'board' games, but I'm getting my bro a copy of Citadels and Wizard for his birthday next week.