Almost all creatures in Tibia give some experience points when they are slain. The amount of experience points earned by killing a creature depends roughly on how difficult it is to defeat that creature. It varies from a Penguin which will give 1 single experience up to the Juggernaut which will give 8,700 experience when killed. There are also a lot different kind of bosses in Tibia, which might wield even more experience (including Inquisition Bosses). There are also some creatures that give no experience at all, those are no challenge to kill since they don't fight back when attacked.
All characters start without experience points when they first enter the game, on a small island south of Rookgaard. By doing The Beginning Quest they get experience, on Rookgaard they can kill monsters which give experience, which can become a very large number. The experience is represented by a characters level. On the official website is the full experience table where is listed how much experience is needed to reach a certain level.
Find out more about your experience by using the Experience Formula.
Occasionally, to compensate for loss of experience due to death or an extended period where players are unable to hunt as a result of DDoS attacks, or even just to celebrate holidays like Christmas, CipSoft may establish a period in which monsters yield double the experience they would normally when they are killed.
So far, this has happened three times. The first time from November 27th, 2007 to December 4th, 2007, to compensate for lag and kicks as a result of DDoS attacks. The second time from March 18th, 2009 to March 25th, 200; to celebrate Christmas, and the final time from April 20th, 2013 to April 30th, 2013; again to compensate players for experience and play time lost due to DDoS attacks.
During Double Experience events, Premium Players with enough stamina still gain an additional 50% experience over the double experience awarded to other players.