The Hellions were a biker gang formed mostly by demons of the same species, as well as a few humans. These demons were humanoids with wrinkly skin, sharp teeth, pointed ears and no nose. In addition, their faces were adorned with tattoos and leather straps attacted to their skin.
The Hellions were led by Razor and enjoyed raiding and looting towns, though only targeted small backwater ones that they considered vulnerable. Even though the gang had human members, vampires weren't allowed due to their inability to go out during the day.
Aside from raiding towns it is implied that the hellions were not above taking pleasure from others. It is also implied that even the things we can't see aren't human in any way. "Now let me tell you something, children. We're not going to fight you. We're just gonna hold you down and enjoy ourselves for a few hours. You might even live through it, except that certain of my boys got some anatomical incompatibilities that tend to tear up little girls. Yeah. So, who wants to go first?" This was said by Razor a little after the 28 minute mark in the bargaining part 2.
They came after Sunnydale when they learned it wasn't protected by the Slayer, but a robotic duplicate. Their raid was shortly taken care of when the real Buffy was resurrected and killed many members.