Minecraft wiki entry for Target_selectors, associated text: @e[family=skeleton] — Select all skeletons, wither skeletons and strays.
@e[family=mob,family=!monster] — Select all mobs that are not also monsters (so for example cows, chickens, pigs, but not zombies or skeletons).
@e[family=monster,family=undead] — Select all monsters that are also undead (that includes monsters like zombies and skeletons, but not creepers or endermen).