PlanetSide isn't your typical MMORPG. Instead of letting you loose in a world where you can do things like tailor clothes or hack away at a rock for hours, you are thrown onto a planet, that's right I said a whole planet, where a war is raging amongst three rival factions. These factions formed after colonists from Earth settled this distant planet named Auraxis. The wormhole that brought them there suddenly collapsed, leaving them stranded to start a new society. So they chose a life devoid of the mundane activities of an Earthly existence and chose complete, world encompassing war.
When you enter PlanetSide, you are given the choice of siding with one of the three factions. Each faction has different equipment, so study them well before you make your choice. However, all three use an alien technique from a long dead race that used to live on Auraxis. This technology allows people to regenerate, so in effect there is a huge war going on where nobody dies. It's like the biggest game of paintball you've never played.
Once you join a faction, you are then left on your own to join player created squads and help your side's effort. There are many different continents on Auraxis, each with plenty of action for everybody. The war that rages on the planet is consistent, so when you and your fellow soldiers take over an enemy base, it will stay in your possession until someone takes it back.
In place of guilds, PlanetSide offers something different. Players can create their own squads called 'Outfits', much like guilds, affording them the ability to better communicate with one another. You don't have to join one of these, but if you stay a loner you'll probably be floating around from group to group and won't really appreciate everything this game has to offer.