Minions are units you summon that fight for you, using their own skills (and not yours). Unlike Traps, Totems, and Mines, they do not benefit from your stats unless those stats specify that they affect minions.
Some minions are persistent and will follow you around, attacking anything near you. Others have a limited duration, and will wander around where you summon them.
Minion stats are, unfortunately, not directly visible in-game. This is one place where an outside utility like Path of Building is almost essential.
In general, Minions are affected by bonuses from three possible sources:
- Stats on you that specify "Minions", like "30% Increased Minion Damage".
- Support Gems linked to the Skill Gem used to summon them. For example, if you link Melee Physical Damage Support to your Summon Skeletons gem, the resulting Skeletons will have their attacks boosted as though their attacks were, themselves, linked to Melee Physical Damage Support.
- Effects that benefit your Allies, which includes Minions. Effects that benefit Party Members, however, do not affect Minions - only other players in your party.
If your minions somehow gain stats of their own (for example, by giving them the bonuses from your shield via the Necromantic Aegis Keystone Passive), they will benefit from those stats just as you would for the most part.
In addition, the actual summoning of a minion is a skill you use, and thus benefits from your stats: for example, Increased Cast Speed will allow you to cast a minion-summoning Spell more quickly.
Most Minions deal either Physical damage or the type obviously associated with the minion (for example, Fire Golems deal Fire damage).
Some minions are Summoned, and can be created at will. Others are Raised (from a corpse) or Animated (from an item on the ground), and require some condition to create.
Most types of Minion have a limit to the number you can summon at once. If you summon more than the maximum number of Minions, the oldest Minion will die and be replaced by a new one. Various effects, including some passives and Unique items, can increase this maximum limit.
Most Minion Skills have a limit only for themselves, but Golem Skills (of which there are several) share a single limit among them (by default, 1 Golem, although it is possible to summon as many as 7 with the proper build).