Summoning

Summoning is the magical art of calling an elementals into physical being (as opposed to atavism which summons the elemental in mental form).
All summoning requires a focus - without one, you might summon something at random. For weaker elementals, the focus might be the air around you, a candle, or a twig, depending on the sphere you're using. More powerful elementals can be summoned without a rare focus, but are more difficult to summon.
For safety, elementals are normally summoned within a containment circle. While nothing a summoned elemental can do is able to damage a properly drawn containment circle from within if it's powerful enough, even so much as a thread or leaf crossing the edge of a circle can break it. This is why most magi summon indoors.
Summoning is a difficult art, and is aided by having an understanding of the sphere associated with their element. A few people can learn to use summoning on its own, but they're rare, and even rarer are those who have soul-bound elementals that grow with their own power.
All the major spheres, aside from energy, allow you to summon elementals. While some minor spheres have elementals that you can summon, not all do - for example the artificing and poison spheres.