The series began as a seven episode OVA series. It was popular enough to be made into a 26 episodes TV series, The Wanderers, set in an alternate timeline. A second thirteen epidode TV series, The Alternate World, is a continuation of the original OVA timeline.
The story focuses on four people from our world who are accidentally transported to the fantastical world of El Hazard, which is threatened with a massive war.
El Hazard is a "light" series -- while it has a fair amount of character development, the series focuses almost entirely on action/adventure and comedy. Contrast this with, say, ''Tenchi Muyo!', which has a bit more character development and drama (especially in the later series).
Historical context: After the phenomenal success of Tenchi Muyo!, AIC wanted to repeat its success. It created El Hazard, which proved that AIC could reliably create popular anime.