PvP means Player versus Player, these are solo or group battles (also known as rallies) against another player’s base or troops. 

IMPORTANT NOTE #1: Players are able to attack one another at any time. Activating a shield around your city is the only way to safeguard your city. Shields only protect the troops inside your city. Troops that are outside of your city can still be attacked, injured and killed.

REALM WRATH 
Attacking or rallying a player, shring or stronghold will disable your ability to shield for a time. Subsequent attacks will reset this period of time if the new time is longer than your current time. This is to prevent players from hitting and immediately shielding, so if you are going to hit people, make sure to have strong dragons on your wall!

The amount of time you spend in Realm Wrath is dependent on the difference in Townhall levels between you and the user you hit.

Realm Wrath Timers:
10 Levels+ lower - 30 minutes
5-9 Levels lower - 25 minutes
3-5 Levels lower - 20 minutes
1-2 Levels lower - 18 minutes
Same Level or Higher - 15 minutes

There are Free for All and other PvP specific events mixed into the game schedule - in order to earn points and compete in these type of PvP events, you will have to attack and defend against other players.