What he said ^
But you forgot something essencial!
Each campaign (besides prophecies and Eye of the north (because it's an expansion)) added 2 new classes to the game
Factions:
--Assassins: Sneaky little guys, primary weapons are daggers and they use hit and run techniques because they have high DPS but low health and low armour too. The hit and run is done by using several skills they have that do "shadow step" (teleport, pretty much).
--Ritualists: They use the power of the spirit world to battle, this means summoning spirits, spirit weapons for you or your allies and also ashes of dead heroes for big buffs. The spirits work kindof like echantments with a physical presence although there are some offensive ones, but, kill the spirit the enchant drops. The spirit weapons are buffs or added abilities to allies weapons. Primary weapon are staffs or rods.
Nightfall:
--Dervish: Holy warriors, they use scythes to cut through their enemies. they have also a vast range of self enchants to last longer in battle. They can also take the form of any of the 5 gods, making them a lot more badass and tough to kill
--Paragon: Angelic protectors. They throw spears at the enemies, and ususally stand on the backlines using shout and chants to support the group. They are very usefull at buffing, but they also deal decent damage.
So that's a summary of the classes, if you just get Prophecies and Eye of the north, you won't get any of these 4, all you'll get are the "core" classes (They exist in any campaign):
Warrior --> Tank guy
Monk --> Healer guy
Ranger --> Ranged non-magic DPS and conditions (poison, burn,...)
Mesmer --> Mind magic, (awesome to cut through monks and make warriors kill themselves, literaly

)
Necro --> Makes undead from dead bodies (Oh so fun in alliance battles, so many bodies to turn)
Elementalist --> Can work with 4 diferent elements Fire(DPS), water(negative enchantments), Air (damage-shutdown), Earth(tank)
Well that said, it's up to you really, but whatever you do, Guild wars is worthy of being played, as it is a really great game (Best combat system in any MMO IMO)