Titans, according to the MotP, may have been the first servants of the primordials (a la angels and gods).
Titans are basically exaggerated giants. They're bigger, they're more powerful, and they express a greater affinity for their element. Storm titans are more, well, stormy, frost titans are more icy, fire titans are more fiery, etc., than their smaller giant counterparts.
The most powerful titans are influential lords in the Elemental Chaos, like Surtur and Thrym.