Experience is what gets you from one level to the next. Every monster you kill usually will give you experience. If more that one person is attacking the creature, then whoever does more damage will get more experience. The harder the creature is, the more experience it will give.
While having a premium character or having platinum on a character, 50% more experience will be recieved (as long as they have full stamina). So, if you were to kill an orc for example, a premium account or an account with platnum would get 75 experience instead of the regular 50.
Stamina prevents people from hunting too much. After a few hours of used stamina, experience rates will go to normal for premium characters and characters with platinum. Stamina maxes out after 56 hours. Stamina helps prevent autohunting.