As I mentioned before Ymer was the first great giant emerging from Ginnungagap. He was not a very nice fellow and is described in the Edda as evil (how one could determine that when he was all alone I do not know.)
Ymer had problem sweating while sleeping, and that caused a man and a woman to emerge from his armpits. His two feet used to party while he slept as well, and they managed to breed a son. Those creatures were not human and they named themselves Hrimtursar but man would eventually call them giants.
The Edda does not draw a clear line between the Giants and the Hrimtursarna. Sometimes they are mentioned together as being the same thing and sometimes they are mentioned separately, but it´s commonly accepted that they are to be viewed as the same thing.