PvP is part of the Diablo 4 end-game and takes place in what are called the Fields of Hatred which are are corrupted areas of Sanctuary.
Born from their contempt of mortals, Mephisto has cursed specific areas of Sanctuary, transforming them into deadly Fields of Hatred. These designated PvP zones are the proving grounds for players looking to bring renown to their name through blood and zeal.
When enterting one of these corrupted areas, players can be attacked by other players and freely and also take part in PvP against any player they are not grouped with. Players will also be able to PvP as a group.
These special zones will include objectives and monsters so players will still have to be on their guard against the evil forces of Santuary.
If a player is killed by another player, the defeated player will drop Seeds of Hatred which can be gathered by the victor.
Should any player die when carrying Seeds of Hatred, the seeds will be dropped for the victor to loot.
Seeds of Hatred are converted to Red Dust at the Altar of Extraction and the Red Dust can then be redeemed at PvP cosmetic and mount vendors for ornamental reqards. Note that once seeds of hatred are converted to Red Dust, the Red Dust currency cannot be looted by other players which means you'll want to convert them as often as possible.
Once at the Altar of Extraction, other players will see you are in the process of changing your Seeds of Hatred into Red Dust making you vulnerable to attack with the potential of losing your seeds before they are converted.
Blizzard has also stated that players who are stomping all over other players in these PvP zones will be highlighted to other players and a bounty could be put on their head. Should you take on the challenge of defeating this prolofic PvPer you may be rewarded handsomely.
Read more about Diablo 4 PvP on the PvP section.