They're from german folklore; basically like elves and most other fairies; ambivalent most of the time, but easily angered. Apparently the element cobalt was named for them, since miners blamed kobolds for how difficult the ores were to work with. They could also apparently materialise in the forms of either an animal, a human, a candle or a fire, so presumably they're also responsible for catsplosions. They were popularised I think its fair to say by D&D.
http://en.wikipedia.org/wiki/KoboldAs to whether they're dangerous; last time I embarked on a kobold site the overwhelming majority were utterly harmless; unarmed, friendly, and willing to remain friendly until you attacked them personally. (i.e., they wouldn't act to protect other kobolds.) Watch out though; I had a thief spawn with them as well, who was armed. He was initially friendly, but became hostile when I attacked another kobold, and promptly killed off six of my seven dorfs, including three at once. The seventh dwarf killed him and then amassed some three-hundred kobold kills to his name though
Also, the kobold cave for me led down into the caverns (which were full of even more kobolds) so you might want to keep an eye out in case something wanders up from there. Tbh, if you don't mind your units list being spammed by kobolds, I'd just steal all their stuff and leave them, unless you want to use them to level up your military.