you should all know it by now. It's the planet you live on. If it is not
the planet you live on, then:
B. You are psychotic.
Earth is the planet from which Makoto, Jinnai, Nanami, and Fujisawa all come from. It is also the place, in the beginning of the story, the 'Wandering' characters are trying to return to, specifically, Japan and Shinonome High School.
The Earth (i.e. Japan) of this story is set in the mid-1990's. So, take into consideration all the social, cultural, and technological developments of that time and you have an accurate picture of where they come from.
While Earth is the Wanderers' original home, and I'm sure some of them would like to see it one more time sometime in their lifetime, El-Hazard has become their 'home.' Characters like Fujisawa and Jinnai have built lives in El-Hazard they would not want to leave. Makoto wants to return to Earth only so he can rescue Ifurita and then return to El-Hazard.