Giants are a type of creature similar to humanoids, but of the very large sort (above 3 meters or so), and larger than most ogres. The category includes most semi-intelligent and some intelligent large humanoids. Most giants are elite.
Many of the giants on Azeroth were created by the titans as part of their ordering of Azeroth, meant to protect and oversee the land and ocean. According to Brann Bronzebeard, they were affected by the Curse of Flesh, but in less scope. Only a handful remain in the most untouched areas of Azeroth, where they are still fulfilling their ancient duties, although some (molten giants and Baron Kazum for example) seem to serve the Old Gods against their will. When Illidan Stormrage threatened the security of the world, archdruid Malfurion Stormrage used his vast powers to call upon the aid of the mountain giants. Kael'thas Sunstrider ordered the dropping of huge crystals onto Outland, which summoned giants and corrupted them. This explains why Draenor's giants seek to harm the lands instead of protecting it.
Characteristics in World of Warcraft
Depending on the type of giant, some speak Titan, some speak Giant, and some speak Kalimag.
List of giants
Other creatures called giants in Warcraft include:
"Crystal Giant" concept art.
"Fjord Giant" concept art.