Experience

Experience (often abbreviated exp or xp) is the numeric stat you need in order to level your character. It can be obtained by completing various tasks like killing mobs.

Cap
A cap is the maximum amount of experience you can get at once (under normal circumstances). As your total level increases, so to will your cap. The maximum normal cap is 250k xp.

Overcap
It is possible to get more experience than your cap by defeating a particularly hard mob for your level. This is rarely happens more than a few times for the same mob. In particular, getting twice your normal cap as a reward is referred to as a doublecap.

Denting
see global exp thacker

Repeatedly killing the same mobs and running the same area will reduce the amount of experience you earn. This is not such a problem for low level characters but becomes more apparent the higher you level. One sign of this is, upon killing a mob, receiving the message

Rotation
In order to avoid the affects of denting, high level players will vary the locations and mobs they run. Low level players may need to rotate between 2-4 areas while higher level characters may need to rotate 15 or more to avoid getting dented experience.

Mar, 2013
In March 2013 dentin published some stats on board 8 showing the percentages of where experience was coming from. 

Sep, 2013
Taken from board 8

Dec, 2015
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Class experience
Most spells and skills will give a variable amount of xp per use with the amount usually denting after successive uses. Below is a list of some of the notable xp areas of the various classes. To some extent all skills and spells provide xp however the amount they give and the amount they dent various considerably.

Warrior

 * combos

Mage

 * brewing potions
 * store power

Thief

 * backstabbing
 * brewing poisons and antidotes
 * making bombs

Cleric

 * turning undead
 * healing in groups

Druid

 * crafting totems, spellstaffs
 * making salves and tinctures
 * carving runes
 * farming lightcatchers

Tips

 * Rotate areas
 * Join groups
 * Use a variety spells/skills
 * Explore
 * Run dangerous aggro areas
 * Perform your primary class's major function (e.g. Clerics can turn undead, heal, etc.)