Simply put, a PVP Faction is a map or area where you can kill other players. You will repeatingly run into them, for you share these maps with the other faction. When in a PVP Faction, it's important on which line you are.
When on Line 1, you CAN NOT be harmed by players of the other faction, or your own. This Line, is entirely safe. Aside from Elites and normal monsters trying to beat you up, if not, kill you.
This Line is mainly used for new players to level up, and for general players that like to play without danger. You also see alot of AFK Miners and Herb collectors here.
Line 1 is still a PVP map in Eligium PH :)
When on Line 2, you CAN be killed by players of the other faction. This gives no warning. You can be ambushed when you try to level, are busy on a quest, or are Mining/Herb collecting. You have to be on your toes, playing this line, for you can be attacked any moment.
This line, however, gives double Soul Points when killing monsters and bosses, in both maps and dungeons (doesn't apply for Fairy Lands). Also the World Bosses ONLY spawn on THIS line. You can go look for them on Line 1, but you can wait forever for they will never show up.
Alot of people make the habit at a certain level, to switch to Line 2 to get some player kills. These get you Honor Points, but only if the person you killed is in your level range. Meaning; If you kill someone that is either lower or higher then you by 10 levels, you won't get Honor Points.
Note: If you're grinding on Line 2, be careful ALWAYS! People can pop up any time near you, and you'll be dead without a clue. Also, DO NOT GO AFK ON LINE 2!! Unless, of course you are in one of the Main Cities. You cannot be harmed there.
It can be fun to go player killing with a group. As a group you're less vulnarable, and you have a larger chance of survival. Even going in a duo or threesome can be fun. Duo's such as Druid/Hunter or Druid/Mage can be very overpowered!