Goblins are a people of small grown, thieving wrongdoers.
They are often seen at the scenes of thievery or robbery because for whatever reason those beings are known for always taking things that won't belong to anybody. They are using the advantage of their nimble and small body that can easily be overlooked to slip through security and into enemy camps.
Although it is not totally clear why, the goblins do have some kind of relationship to the orcs. Whereas the orcs are brutal and somehow a bit dumb, the goblins are able to trick their enemies and they are very nimble with attacking. Due to their nature as thieves, they have a poor reputation at almost every other race. They even tend to steal from giants as shown during Huge Misery with the giant Mjörn.