Giants (also called etins or thurs) were typically the enemies of both men and gods. However, this was not always the case, and some men and gods even mated with giants. For example, Loki is probably the offspring of a giantess and a god.
Giants of Note:
Hrímgrimnir
Hrym/Hrymr - Leader of the giants, he accompanied Loki at Ragnarök.
Suttung
Utgarð-Loki - Ruler of Utgarð.
Ymir - The first giant.