Titans are giant world bosses that are defeated with large platoons of players. Currently there are three and serve as end bosses in raid missions: Necronus, the giant spider boss from the beta; Baneclaw an even more giant terror claw; and Kanaloa, a giant magma worm that spits fire.
Titans were previously in the beta of Firefall and were open world bosses that would invade many of the establishments. The previous Titans were giant spider-like creatures. The open world Titan invasions were removed due to the fact that they didn't function well on live servers and because the developers at Red5 didn't like how it functioned in the game. Many of the towns still have Anti-Air guns built into their structures, although they are now only ornamental.