Earth is the third planet in the Solar system.
Earth, sometimes referred to as Terra, orbits the Sun, also called Sol, and has one satellite devoid of life, called the moon, or Luna. It formed approximately 4.5 billion years ago, and life has existed here for at least 3.5 billion years. Currently, it has several large land masses including North America, South America, Europe, Africa, Asia, Australia, and Antarctica. The rest of the planet, some three-quarters of it, is covered in oceans.
Earth is home to the Three Races: gargoyles, humans, and Oberon's Children. In the past it was also the home of the Lost Race.  It contains a plethora of plant, animal, and other life. Gargoyles in particular are connected to the natural bio-rhythms of the Earth as it governs their sleep and breeding cycles. The magical island of Avalon is also tied in a nexus of magic inherent to Earth. 
While Earth is the only known planet in the solar system to ever provide life (sentient or otherwise), within the greater Milky Way Galaxy there exist aliens such as the N'Kai, the Space-Spawn, and another race involved in their war.  However, life-bearing planets are apparently quite rare in the Milky Way, hosting less than 100 extraterrestrial races (of which fewer than twenty are involved in some degree in the current intergalactic war).  In considering the size of the universe itself, there is the possibility that there are magic-based races originating from planets other than the Earth, but the prevalence of this phenomenon (even within the Milky Way) has yet to be revealed. 
- Earth at Wikipedia, the Free Encyclopedia