The Garden of Eden was a place on Earth, created by God for the original humans. They resided in the garden, before Lucifer corrupted their minds, and they were cast out. After organisms evolved into fish, fish evolved into apes, and apes evolved into humans, God created a paradise on Earth called the Garden of Eden. He placed the humans Adam and Lilith in the Garden, and declared them his favorite creations.
He told all the angels in Heaven to bow down before the humans, and serve them. Lucifer refused to bow down before a species that was younger, and inferior to him. Because of this, he turned his back against God and no longer wished to follow him, banishing himself.
When Lilith would grow tired and angry at Adam, refusing to submit to his desires like she is a mere play thing, the archangel Samael took the opportunity to take Lilith for himself, which eventually created the sin of Lust. Because of this, God cast out Lilith and Samael and eventually replaced Lilith with Eve, so humans could continue to reproduce, and his favorite creations could live on. The one rule they had to follow, was that they could not eat from the forbidden fruit.
Lucifer, however, eventually made his way to the Garden and disguised himself in the form of a snake. He then corrupted their minds, and tricked them into eating the fruit in order to prove to God that they were truly imperfect and that if they were, they would never have listened to him. This sparked the War in Heaven where Lucifer declared war against his father and brethren, and was eventually cast out. God cast the humans out of the Garden for disobeying him, and created a realm called Hell, placing Lucifer, Lilith, Samael, and the fallen angels who followed Lucifer during the war there.