Minion masters (or minionmancers) are Necromancers or Ritualists who specialize in maintaining large armies of animated undead creatures, usually bone minions and bone fiends. Such characters are popular in PvE as questing and farming characters. They use minions to deal mass damage and to defend soft targets like monks. The maximum number of minions per Minion Master is 2 + 1 for every 2 ranks in Death Magic.
Due to the maximum number of minions tied to the necromancer's Death Magic attribute, Primary Necromancer Minion Masters are much more common, due to access to Runes and Headgear to raise their attributes. A good number of Ritualist skills have effects that trigger off of or enhance summoned creatures, such as Boon of Creation and Explosive Growth; most of these skills are in the Ritualist's Primary Attribute, Spawning Power, making Rt/N Minion Masters also popular.
For a general guide to playing minion masters and a selection of minion master builds, see the general minion mastery guide.