Summoning is a loosely defined term that is used by non-mages to describe all branches of magic that are used to call beings - usually as support in battle, but sometimes as aid or servants. The techniques of different kinds of magic grouped together as "summoning" have little or nothing to do with each other.
Main article: Demonology
The most commonly encountered branch of summoning involves calling demons to the physical plane by some magical means. Depending on the power of the demon that is summoned, this can be as easy as a short formula spoken to call a lowly imp to battle, or a complex ritual requiring many months of preparation to summon an arch-demon like Grah-Hoth to this plane.
Main article: Necromancy
While the fields of necromancy and summoning overlap, not the entire field of necromancy is referred to as summoning. There is a part of necromancy that involves animating a dead body to create undead, and another part that involves calling dead spirits from the netherworld to manifest themselves on the physical plane. Although both branches use very similar magic, only the latter is called summoning due to its effect of manifesting creatures out of thin air.
There are spells that create usually insubstantial shapes that are neither demons nor undead spirits. While these illusions are practically useless in combat, they serve perfectly as distractions to cast in the way of enemies. Some illusions are even capable of manipulating solid objects or physically injuring people, but most of them are easily dispelled.
This branch is unrelated to the others in that it is the only magic that does not call into existence beings that did not previously exist on this plane. Instead, a powerful beastmaster is capable of calling out to any animals that are nearby, summoning them to his aid.
Since this magic requires an animal to be already within calling distance (powerful mages can call animals from further off, but it will take time for the animal to actually arrive), and not physically blocked off from the place the mage is calling from.
Although mundane beasts are less powerful than the otherworldly beings called by nether summoners (a collective term for necromancers and demonologists), they are real creatures and cannot be dispelled - although the beastmaster's control over them may be broken.