From the Creator's Page:
GIRLS is short for "Grymt Innovativt Roligt Litet Spel" (Hideously Innovative Fun Little Game) and that's precisely what it is. It's a series of small, simple, fun, action-packed and imaginative role playing games. All of them share the common trait that they are about supernatural teenage girls who battle the various problems that a high school may present.
Each of the RPGs is a stand-alone and you don't need the other games or the GM manual to play them. But it will of course be more crazy and fun if you do.
All of the games in the GIRLS series use the rule system KYSS (Krilles Ypperliga Spel-system) that made its debut in Playelf. It's a well-tested and trustworthy system that works well in crazy situations.