The Giants of Jotunheim are enormous superhuman beings, most of whom dwell in the other dimensional realm of Jotunheim, one of the Nine Worlds of Asgardian cosmology. They are the traditional enemies of the gods of Asgard, with whom they have gone to war numerous times over the millennia.
The Giants are basically humanoid in appearance with their most distinguishing feature being their height. Per mythology and Marvel Universe Wiki, the average Frost Giant is twenty feet tall, although some reach up to thirty feet. Why then did they get shrunk down to about 8-10' tall during the movie?
More on Marvel.com: http://marvel.com/universe/Giants_of_Jotunheim#ixzz2zTY50uaZ