AddictedtoGaming.com has a great overview of PlanetSide up:

PlanetSide is a MMOG (Massively Multiplayer Online Game) made by Sony Online Entertainment (SOE). It is an online only, subscription based cross between a First Person Shooter (FPS) and a Roll Play Game (RPG). Based over 1000 years into the future, PlanetSide takes MMOGs to a new level, allowing players to wage war on a planetary scale.

The story behind the game, although somewhat simple, is believable and exiting. Over 1000 years ago a wormhole to a distant planet was found. Over the last several decades the Terran Republic have been colonizing the new planet, named Auraxis. They established three main bases and also countless smaller outposts and bases across the planets ten continents. While building these bases, remains of an ancient alien race, the Vanu, were discovered deep within the planet. The fate of the Vanu could never be found, but their remaining technology allowed the feeble earthlings to “rebirth”, effectively wiping out death. During the late stages of the colonisation of the planet, the wormhole collapsed, leaving the Terran Republic expedition force stranded. Over the following years tensions between the stranded forces grew. Now realizing that they were probably cut off permanently, two groups of colonists split from the Terran Republic. They formed their own empires, based on their own beliefs. The Vanu Sovereignty and the New Conglomerate were formed. The three empires couldn't agree and eventually war broke out. A war for the control of the planet Auraxis. A war which could never end. A war without permanent death.