You can also pvp from any of the campaigns. As already stated, everything is online and you link the games together so all three are busy with players and trading, etc. Read the posts about the different games because as you can see it is all a matter of opinion.
Prophecies was the original game and most agree it was the best. Factions moves your character along very quickly and gives you the options to do alliance battles. In my opinion it also had the best end game reward although to be honest it was my least favorite of the games. However, the ritualist is an awesome character so it is still worth it. Nightfall is better than factions as a game itself (imo) and the heroes are one of the best additions to the game in a long time. Heroes are henchmen that you can set skills and secondary professions for, once your character unlocks them you can use them most anywhere.
I believe there is a free trial that you might check out...think there is a link on the home page here, you could look at all three and see what you think.
Good luck and welcome to guild wars
