Hey just a quick question, is a forum skin basically the background of the forums :3
thanks.

It can refer to any combination of an avatar, signature or background. Can also be known as a forum set. Backgrounds and sig-pics are only available if you buy them (if you didn't know).

A forum's skin, in the general sense, is its theme.
i.e. This board shows a black bar above each post, with the time since the post on the left and the chronological position of the post on the right. Then on the left of the postbit is the username, usertitle, join date, etc.
Basically, it's the layout and colour scheme.