Earth

"This world is older than any of you know. Contrary to popular mythology, it did not begin as a paradise. For untold eons demons walked the Earth. They made it their home, their... their Hell. But in time they lost their purchase on this reality. The way was made for mortal animals, for, for man. All that remains of the Old Ones are vestiges, certain magicks, certain creatures"

- Rupert Giles

Earth is a hell dimension which became populated primarily by humans after the Old Ones were banished. However, it remained a home to various supernatural creatures such as demons and vampires. Many human warriors, such as the Slayer, spend their lives protecting Earth from apocalyptic threats.

History
"Either the Earth was an improvement, a step up, or it was a ghetto. Either way, it was on its own. The Seed, the source of all that magic, was the only thing powerful enough to keep it from bleeding back."

- Spike

Earth was born from the Seed of Wonder, the source of all magic. It orignated as a world of demons, and may have been "spillover" from another, far more horrific dimension. The Seed brought the demons and magical energies through to Earth and kept them there, acting as a sort of "cork" in a bottle, so long as it remained in place at the Hellmouth which would eventually become Sunnydale, California.

"Yes. In the beginning, before the time of man, great beings walked the Earth. Untold power emanated from all quarters—the seeds of what would come to be known as good and evil. But the shadows stretched and became darkness, and the malevolent among us grew stronger. The earth became a demon realm. Those of us who had the will to resist left this place, but we remained ever-watchful."

- Jasmine, one of the Powers That Be

Higher beings such as the Powers That Be also existed on Earth at the beginning of time. However, eventually the Old Ones rose up as the dominant race and the Powers That Be fled to other dimensions. The Old Ones ruled Earth for an unknown period of time known as the Primordium Age.

However, the Old Ones were eventually overthrown by a new race; humanity. Humans managed to restore the balance between good and evil and banish the Old Ones to other dimensions.