When you attack a player, you become in PvP Mode, in this status you will have some different restrictions, rewards and penalties. There is 9Dragon PVP mode guide.if you are interested in it .just look through.Hopefully the article can help you. I believe you will learn something from it.

You can see you are in PvP mode in your screen with remain time you will be in that status.

To attack a player you must select him, and: 

 If he is vagabond or from same faction than you, you must press Shift key in your keyboard and act like your enemy is a mob.
Once you are in PvP mode you don’t need keep pressing shift or press it again.

If he is from the opposite Faction you can attack him like a mob, and don’t need press Shift.

When you select a player, you will see if he is from your faction or vagabond (grey name bar) or from the opposite faction (red name bar)

During the time you are in PvP mode you cannot:
To use Lightfoot Technique. 
To use Summoning Techniques.
Revive on the Spot, or Reviving Chi Kung.
To log out or quit the game
Also, during PvP mode your skills will be weakened.

You cant’ attack to a player who is in your party, or who is in same Hero Band, Storm Party, Raiding Party or Guild.
And you only can attack one member of your Clan if you (or he/she) are wearing mask.

To exit from PvP mode you must stop to attack your opponent and wait some time, it will be different depending on the actions you have done:
If you have not hit to your enemy you need wait 5 seconds
If you have hit your enemy you must wait 20 seconds.
If you have killed a player you need wait 60 seconds.