Ha, I was going to point you to a definitive place where someone has already explained, but I could find anything! So, here\'s my own interputation.
First of all, there is no fighting in the current demo, MB. Soon, CB will be released, and it will start the fighting- but wheither PvP will be implented right away or not is still unknown.
Eventually, there will be limited PVP. players can offer to duel each other, and if the second party accpets, they can now go at each other. Also, entire guilds can offer duels, but since there are many people going at it, there are some details they we won\'t know about till we see it. There has been mention of each guild have a score, that by getting someon to admit defeat you go up.
Also, there will be open PvP areas, like an arena, or possible an extra dangerous area, where you are told as soon as you enter that you are at risk from other people.
To sum it up, you can fight other people if you go out of your way to find willing people, but no one can force you to fight.
Oh, and further more, if someone is causing yo trouble, constantly trying to get you to dual, tricking monstors into fighting you, etc, there will be GMs who will be able to help you.