XP

You gain XP by killing enemies and playing objectively. Finishing matches give you bonus XP and you can also gain XP through Achievements and by bying Items.

Leveling Up
You can level up multiple things. When you gain XP you level up both your profile and your different classes. You also gain XP for your weapons by using them. Leveling up your profile unlocks different weapons and upgrades to be bought and leveling up classes gives you weapons, gadgets and clothing for free.

Sources of XP
XP can be gained in multiple ways, as said before. Below are listed all kinds of getting XP:

XP per action:

 * Kill: 100
 * Elimination Kill: 500
 * Champion Kill: 1000
 * Kill as Champion or Slaughterer: 50
 * Kill with Annihilator: 125
 * Kill the Flag Carrier: 300
 * Assist: 25-75 (Depends on how much damage you dealt on the enemy before he was killed)
 * Flag Pickup: 50
 * Flag Capture: 150
 * Point Capture: 200
 * Bomb Carry: 150

Attributes that gain you more XP on kill:

 * Headshot: +25
 * Longshot: +25
 * Pointblank Shot: +25
 * Melee Kill: +10
 * Kill a Capturer: +50
 * Kill a Defender: +50
 * Kill while holding the Flag: +75
 * Kill while Capturing: +100
 * Save a Teammate: +25
 * Save yourself: +25
 * Dominate an enemy: +50 (kill him three times in a row without him killing you)
 * Payback: +50 (kill the enemy who dominated you

Special Gains of XP

 * 3 Kill Streak: +25
 * 5 Kill Streak: +50
 * 10 KIll Streak and every 5 kills streak after that: +100
 * Secret XP Box: +1,000