That's us! We're the meanest gang in town. Actually, we're the only gang. But nobody messes with us.”— Sherry
The Skulz are a gang in Junktown.
They do Gizmo's dirty work, and basically anything else they want. Vinnie has a deal with Killian Darkwater - the Skulz do not bother Killian and he does not bother them as long as they keep it low-key. The Vault Dweller can put a stop to their activities for good in a quest given by Lars.
Their name is a pun to the word "skulls".