PvP

Fighting another player is consensual: you cannot attack anyone who has not been set as your enemy, and who has not set you as theirs. An attempt is stopped, and the game says why.

Who cannot fight whom

Party members. Never, in any circumstances.
A hardcore character and an ordinary one. Blocked in both directions.
Inside the arena, both players must be in the same pit.

What a kill is worth, and what it costs

A player kill counts only if all of these hold. If any one of them fails, the victim is raised on the spot with no penalty, and the killer's party is told the kill was not valid and earns nothing:

the victim is level 10 or above
the victim has no XP debt
the two were hostile to each other
for a party kill, the victim is within the party's level range - see Parties
for a solo kill, the victim is within your level range, or a higher level than you
no free resurrection event is running

Where the kill counts, what the killer earns depends on the levels. A solo kill of a higher-level player outside your level range pays the most. A solo kill of a lower-level player outside your level range pays nothing, and the victim gets a free resurrection.

A valid PvP death is an ordinary death - see Death for what that means and Resurrection for what coming back costs.

Every player kill is reported to the staff.

The rules around a fight

The game stops you attacking or casting harmful spells at anyone you have not marked hostile. You mark a player hostile from the PvP functions in the Player Menu or with the hostile button on the players list. The staff enforce the rules below.

You may only set someone hostile if they are in your PvP range - except in four situations, which you have to be able to show if you are asked:

they are behaving badly towards you;
they are taking your kills, or looting your corpses, without asking to join you;
they walked into an area where you were already hunting;
both sides are in parties and both parties are entirely in the fight.

A known thief - someone who has pickpocketed several players - can be set hostile by anyone those players ask for help.

After you kill someone within the rules, you may not touch them again for 12 hours, unless they start something with you first.

What counts as starting it

Any of these is a PvP interaction, not only an attack:

attacking, in melee, at range or with a spell;
pickpocketing;
taking loot, whether from inside a party or outside it;
taking someone's kills from outside their party;
insulting them.

Logging out straight after pickpocketing is not allowed. The other player must get a fair chance to come after you. It is punished severely, as is logging out to avoid dying - see Conduct.

Organised PvP

Queued matches are on Battlegrounds. Deaths there are not ordinary deaths: there is no corpse and no penalty, and you press Respawn.