Crysis: Warhead takes place during the exact timeline of Crysis, but focuses on Pshycho's side of the story instead of Nomad's. Gameplay is basically the exact same as Crysis, the only change is that you get a few different weapons. It uses the same engine, so it looks just as good and plays just as good.
It's a great companion piece to Crysis plot-wise, and gives players another chance to revisit the island. Just don't expect much of a difference in gameplay between the two.