Slayers is an Anime series based on a novel that parodies RPGs with the story of Lina Inverse and Gourry. The stories take place in a medieval style world (mostly), with magic of course. Most of the show goes in cycles starting out severely humorous, then slowly working towards serious until at the end of the cycle the fate of the world (of course!) hangs at the balance. Until the very end of each of these cycles though, even the most serious moments will usually have a few short comedic relief moments.
Slayers ran for three seasons. Season 2 goes by the name Slayers Next and season 3 is called Slayers Try. There are also numerous OVAs and movies in which Lina is partnered with the well-endowed and somewhat obnoxious sorceress Naga. These adventures take place before the events of the three television seasons. The anime Lost Universe takes place in another dimension of the Slayers universe. There are also a few manga series based on the Slayers universe.
Warning: plot details follow
In the universe of Slayers, there is a supreme being, neither good nor evil, called Lord of Nightmares or Sea of Chaos, which created everything. On the Sea of Chaos, there are 4 (known) pillars. Each pillar supports a world on it. On each world, there are gods (shinzoku) and demons (mazoku), fighting the usual good vs evil war. Would the gods win the war in a world, that world will be in peace. Would the demons win the war, that world will be destroyed and returned to the Chaos.
In Lina's world, the supreme god is Ceiphied, and the supreme demon is Shabranigdo (aka Lord Ruby-Eye). Long time ago, Ceiphied won the war, and cut Shabranigdo's body in seven slices to prevent him from resurrecting. However, Ceiphied was so exhaust that he sunk in the Chaos Sea, leaving four parts of himself in the world.
A millennium before the Slayers stories, one of Lord Ruby-Eye's parts (which was sealed in the body of Lei Magnus, a very powerful sorcerer) came back to life and started the Resurrection War against one of the parts of Cephied, the Dragon King of Water (aka Acqua). Ultimately, Shabranigdo won, but Acqua frozen and sealed him at the Kataart mountains.
More to be added
Warning: incomplete list
In Slayers, spells are put in categories based in the origin of their power:
Shamanistic spells are spells whose power comes from the sorcerer: they don't borrow power from any god/demon. They are the every-day spells, and their power depends on the sorcerer's conditions (i.e. a sorceress can't cast powerful spells in her menstruation).
This spells work altering the nature elements, such as water, fire, air, earth, lightning, etc. Fire is by far the most spectacular, but it could be not the most effective.
This spells attack the spirit of the target, making them useful to attack mazokus.
This spells are intended to cure and purify people. They aren't related to gods as black magic is related to demons, see divine magic below.
This spells borrow their power from a higher being, either a god or a demon. If the caster is not capable of casting the spell correctly, it can go out of control and even consume the caster.
This spells borrow their from mazokus:
This spells would borrow their power from shinzoku, but the divine magic was lost during the Resurrection war, a thousand years ago. However, some priests can use a bit of that lost power.
This spells borrow their power from the Lord of Nightmares himself. They are so exhausting that only can be cast by very powerful humans (i.e. Lina), and can even destroy the world if misused.