You know, there was never really a explanation. It just seems like he is this weirdo that blows things up with his mind! It is weird that he doesn't talk... He is definitely not human. I guess we will never really know!
Well, we do know he firebends with his mind. About the cyborg part, he found out he could firebend with his mind when he was a kid, and he accidentally blew off his right arm and leg, which is why he got new prosthetic limbs. The eye on his forehead is is weakness, because when Toph first hit his eye, it stopped working for a while, then when Sokka hit his forehead again with his boomerang, it caused a backfire, he exploded, and... you know what happened next.
Hope it helped!
Remember, ATLA & LOK ROCK