Exact nature of their type is not specifically in Quran, but probably they are humans or a branch off shoot from humans. But this part is not clear.
What we know is few things through Quran:
1. they are in isolated location and there population is growing there.
2. when they are broke lose, they will come as swarms. Swarms are found in nature, and following two videos will give you some idea about swarms.
http://www.youtube.com/watch?v=kcArEEvQZ-M&feature=related
http://www.youtube.com/watch?v=70SRHG0RRnI&feature=related
3. Swarms are devastating even by smaller creatures like grasshoppers and birds; but swarms of a bigger creature like them, everything will be devastated in their path and this is to which Quran paints the picture of. And people in denial will see their fault in their devastation.
Complete verse in the Quran is as follows: