Iron giants are a type of giant. They are allied with the iron dwarves and iron vrykul in the service of Loken, their creator. As such, they are at war with the frost giants, earthen, and frostborn.
Stone giants, such as Archavon the Stone Watcher, are another form of giant and are often found enthralled by Loken.
Despite the defeat of both Loken and Yogg-Saron, there are still many iron troops throughout Northrend. Thorim and Mimiron are looking for a way to "fix" them.