Slayers is a series about swords, sorcery and a girl named Lina Inverse. Lina's definitely not your typical young lady: she's ruthless, powerful, mean, greedy and above all else... she's gluttonous! Actually, she's not as bad as the rumors about her say she is. She's a caring person but she just doesn't show it.
The Slayers story was originally a series of novels written by Hajime Kanzaka and illustrated by Rui Araizumi. There are two different story lines: one where Lina is traveling with the busty and obnoxious Naga and one where Lina is traveling with the seemingly dimwitted Gourry. The novel stories about Lina and Naga's adventures were adapted into its own manga series as well as a series of OAVs and movies. The stories about Lina and Gourry's adventures, which take place before her adventures with Naga, also have been adapted into a manga series, a TV series and a short, thirty-minute long movie. The manga was based off of the anime.
The TV series is three seasons in length and is the most recognized media form of Slayers. The first two seasons were adapted from the novels while the third season was created by the producers of the anime and does not appear in neither manga nor novels. Valgarv appears in the third season, Slayers TRY, and is an anime-only character.
Currently, the novel series has been completed and there will be no continuation of the TV series. However, a brand new manga series called The Knight of the Aqua Lord is being published as we speak in Dragon Magazine in Japan. So even though Slayers seems to have died down in popularity, it's still going strong!
For more information about Slayers, the world it takes place in, races, the spells available for the characters to use, the anime, manga and novels, I suggest moseying on over to Slayers Universe.