We know not all players like to combat other players. When you join the game your PVP status will be set to enabled. This means you will be able to attack another player who also has PVP enabled.
If another player does not have PVP enabled, you will not be able to attack them.
To enable or disable PVP use the command /pvp.
Tamed animals are also protected from being killed by other players whilst your PVP is Disabled.
You will not be able to turn off PVP for 5 seconds once you have enabled it. This stops unfair combat for players who do enjoy PVP.