Summoning is a form of magic that creates a physical creature to do the caster's will.
Summons are completely mindless, relying completely on the caster for direction. A summon feels no pain from weapon strikes and is extremely difficult to destroy directly, but vanishes instantly if the caster is killed or sufficiently distracted. Several summoners can join their power to create a larger and stronger summon than each could alone.
Humans and possibly arch angels are capable of creating actual living beings. The advantage of this is that conjured creatures need no direction and persist if their creator is incapacitated; the disadvantage is that the technique carries a risk of the creation turning on the caster or causing other problems.