Elementals are beings that walked the world in the time before the Sealing. What they looked like and what they got up to and is largely lost in myth; some don't believe they ever existed. (Spoilers - they do). Stories are told of monstrous beings, walking corpses, capricious tricksters and savage demons.
Some ancient writings talk of the (literally) legendary art of summoning and bargaining with elementals. Obviously, this has never actually worked when tried, but the following is supposedly how it would be done if possible:
Summoning is typically done at a summoning circle with a ward that would be raised before summoning, preventing the summoned elemental from affecting the mortal realm. In cases where the summoner wanted to affect the world outside the ward, the elemental would need to be released from the circle by the summoner. An elemental could theoretically be summoned without a circle, but to do so would be much more dangerous. The creature would not be bound by any rules and could attack the summoner, or leave. It's rumoured summoning circles are magically active places, and spending extended periods of time inside one when not performing a summoning or other work of magic can have deleterious effects.
To summon an elemental, one usually needs the creature’s name. It could be possible for a summoner to specify what sort of elemental they want if they’re not too picky. In desperate cases the summoner might simply specify what service they want, but this is risky (any entity willing to perform the service could turn up, with any agenda).
All stories of a successful summoning agree on one element and that is sacrifice. Pacts made with elementals would always require the summoner to sacrifice something of themselves dear to themFAQ.
If players want to summon an elemental, the refs need to be informed a short while in advance. The outcome of the endeavour will largely depend on the roleplay between the summoner and the elemental and the bargain the summoner attempts to make.
Curiously, summoning is one of the few skills you can perform while Terminal.