In PVP servers, any character can attack another. Only the bank and the office in the beach are safe zones. PVP servers do provide more XP/Gold/Luck for the adventurous.

You lose a dynamic amount of XP when someone defeats you in PVP. It's 2% to 4%. You can also lose gold if you are carrying gold. At level 1 the maximum amount is 100 gold, at level 75, the maximum amount becomes 1,000,000 gold. ^ Marked items can also be dropped on defeat with a %.

Currently the Arena with Irradiated Goo's is a PVP Map on all servers. Same PVE penalties apply in PVP zones.

If you don't want to risk loss, you can safely try PVP with others. To initiate a duel, you need to use the "/challenge" chat command. "/challenge CharacterName" challenges a character, if they are in a party - their party, to duel.