Okay, if you aren't a gamer like me, you've probably read my raves about Guild Wars and thought, "Guild what?"
Guild Wars is a massively multiplayer online role-playing game, or MMORPG. "Massively multiplayer" means that lots and lots and LOTS of people are all playing it together. "Online" is pretty straightforward: it's played, well, online. And you probably know what a role-playing game is; think "Dungeons and Dragons," the most famous role-playing game around. (And no, D&D is not Satanic. And no, it does NOT drive people to suicide.)
Guild Wars is a fantasy RPG; it's not set in our normal world. Instead, it's set in a world where magic is real and a human can get in a fight with a huge troll and win. (Hey, it's a game, remember?) You fight on the side of good, campaigning against evil. This usually involves killing lots of bad things and traveling lots of places.
So, let's say you have a mission to clear out all the bad guys from an area. You have two professions; your professions determine what type of stuff you do. If you're a warrior, you run up to the bad guys, choose someone to attack, and start whacking it with your sword or axe or hammer, until one of you is dead. If you're a ranger, you may turn it into a pincushion with your bow or send your pet in to fight it.
Elementalists have lots of fun: throwing fireballs, hurling ice shards, shooting lightning, or knocking down enemies with hurricane-force winds. In general, you make them miserable through the wonderful world of the elements.
Perhaps you play a necromancer; then you would curse them to steal their strength. And if you are a necro, you can make nifty little minions out of their corpses, which you can use to fight the other bad guys! Isn't that nice?
Of course there are healers: monks, in this case. But monks can also smite. I guess Balth doesn't much like the enemies...
But if you're me, you're probably playing a mesmer. I love mesmers! They have highly trained minds, and use them to do most of the work. They may hex their enemies and give them migraines, so they can't cast spells or even move very fast. A mesmer may follow that up by interrupting their actions, stealing their health or energy, or turning them into pacifists. It's fun, but it's also one of the hardest choices.
Anyway, those are the core professions. The other campaigns add other professions, but those six make up the bulk of every party. Try it sometime; just don't start as a mesmer like I did...