Unsummon, along with the self-explanatory skills, Attack and Throw, make up the basic skills that every new Diablo II character starts with. Using Unsummon destroys a minion previously summoned by your character. By hovering over a minion with this skill active, the status and immunities will also be listed. For instance, the Iron Golem will be shown to be immune to poison, raised skeletons will have the status of undead, and so on.
Unsummon is automatically learned by a character if they have a permanent or temporary skill that allows summoning any creatures. Necromancers and Druids have it by default, while other classes, for example, Amazons, gain it when they learn the Valkyrie skill.