The planet Earth/Terra is the third planet in the Sol/Terran system, located in the Sirius Sector Block. The Human/Terran race evolved here, and it's the "capital" and birth-place of the United Federation of Planets.
Being an M-class planet with a rock-satellite named "the Moon" or "Luna", Earth is a planet with a very variable environment, from hot and infernal deserts to tropics, green forests, and enormous glaciers on the two poles of its rotation. Its surface is covered with 60% percent water, but humanity and other members of the Federation have populated the planet at an enormous rate.
The Federation has had its capital on Earth for over two hundred years, the democratic government of the Federation is centered in Paris on the Euro/Asian Continent. Starfleet Academy is also located on Earth in the city of San Francisco on the North American Continent, each year receiving more and more cadets as the Federation integrates more species. The Federation is also a silent protector of less advanced civilizations within its borders. Though protected by the Federation, these civilizations remain unaware of the protection until they are advanced enough to achieve warp travel, only at that time does the Federation/Starfleet make first contact.