The Section is a secret organization within the Secret Police (Säpo). Blomkvist calls the Section "the Zalachenko club" at first, because it seems devoted to the protection of Zalachenko and his secrets. The Section has, in fact, been protecting him for decades and is largely responsible for Salander being committed to a psychiatric institution and deemed incompetent.
The Section is mostly made up of guys in their seventies, the same basic age as Zalachenko. Originally, it was headed by Evert Gullberg, who kills Zalachenko and himself in this novel. The Section also kills one of its own members, Gunnar Björck (Zalachenko's original contact in Sweden), to make sure Björck doesn't reveal any of the Section's secrets. Gullberg leaves behind Fredrik Clinton, Hans Von Rottinger, Georg Nyström, and several other members of the Section, most of whom are arrested during Salander's trial.