As the Main Page states the game content chiefly focuses upon the Altherion solar system, and the planet Euphoros, but there are also a wide range of other solar systems. Some of which have yet to be explored.
It is not required, but most characters originated from Alternate Earth, called simply Earth in the game. Alternate Earth has the same makeup as our own planet, we even share much of the same history, altered only to fit the game.
However, Alternate Earth differs greatly from our Earth in many ways. The main difference is that Alternate Earth is comprised of many mystical races that do not exist in our world. The second being that Alternate Earth has been destroyed.
The game picks up 1,000 years after Alternate Earth's destruction in the year 3516. Alternate Earth was also a more technologically advanced society than ours, as its final demise occurred in in 2516, roughly 500 years from our time.
More details about what happened to Alternate Earth can be found here.