You can get XP (experience) for shooting enemies,doing missions or killing other players in PvP . The amount of XP you get for a mission is shown in the description. The amount of XP for an enemy depends from your level and the enemy level. Important for this game is: You get the most XP for shooting an enemy around your level.
You will get less experience when the enemies' level is substantially higher or lower then yours. For example, if a player with level 10 shoots an enemy that is level 15, the player only gets maybe 5 XP. If the player has also level 15 and shoots the same enemy, he gets maybe 150 XP. If the player has level 20 and shoots the same enemy, he will get 5 XP again.