Klesa (Sanskrit, also spelled klesha; in Pali, Kilesa) are the afflictions or defilements that keep us from seeing reality as it is. You could think of the klesas as mental disturbances or unhelpful psychological habits.
The three primary klesas are the Three Poisons — greed, hate and ignorance. All the other defilements flow from those. In Theravada Buddhism, the most severe klesas are associated with the demon Mara and sometimes are called (in Pali) Kilesa-Maras.
Over the centuries various sages have proposed different lists of klesas, so there isn’t any one Official List of Buddhist Klesas. For example, the Pali Abhidhamma lists ten kilesas — using the Pali terms, these are greed (lobha), hate (dosa), delusion (moha), conceit (mana), wrong views (ditthi), doubt (vicikiccha), torpor (thina), restlessness (uddhacca), shamelessness (ahirika), and recklessness (anottappa). But in the Sutta-pitaka and in Mahayana scriptures there are other lists.
Beyond the Three Poisons — which you probably will hear about a lot no matter where you practice — the klesas are spoke of more in some traditions that in others. Depending on where you study and practice, you may hear about them a lot, or you may never hear them mentioned at all. These overlap a bit with the Five Hindrances, and like the hindrances, a review now and then doesn’t hurt, especially if you are going through a rough patch.
The defilements presented here are common ones taken from several sources. The names are Sanskrit; if the Pali differs from the Sanskrit that will be noted in the definition. Terms are in alphabetical order rather than order of importance.
1. Ahrika, Shamelessness
(In Pali, ahirika) No sense of shame or conscience.
2. Anapatrapya, Recklessness
(In Pali, anottappa) A lack of propriety; a lack of regret over misconduct. Uninhibited in regard to how one’s actions might harm others.
3. Auddhatya, Exciteability
(In Pali, uddhacca) Flightiness; a tendency to be easily distracted.
4. Avidya, Ignorance
(In Pali, avijja) Avidya is blindness or being unaware of the truth of reality. It is the first link in the Twelve Links of Dependent Origination.
5. Dvesa, Hate or Aversion
(Also spelled dvesha; in Pali, dosa) This hate, anger or aversion is one of the Three Poisons.
6. Kudrsti, False Views
(In Pali, micchaditthi) Views and opinions that are tainted by the Three Poisons. The most damaging of these are a belief in a self (atman) that is either eternal or is annihilated at death. Kudrsti can also be a view arrived at through speculation, something the historical Buddha warned us about.
7. Lobha, Greed
Lobha is also one of the Three Poisons. In particular, lobha is the sort of greed that chases after possessions for gratification.
8. Mana, Arrogance
Arrogance or conceit. In particular, mana is a compulsion to be top dog; to have a higher status than everyone else.
9. Moha, Ignorance or Delusion
Moha and avidya are close to being synonyms. Moha is the primary klesa; the one from which all others flow. It is ignorance of the nature of reality and a belief in a permanent, individual self.
10. Pratigha, Hatred
(In Pali, patigha) Hostility toward other people; a sense of frustration with others, a tendency to find fault.
11. Raga, Greed
Also passion or desire. Associated with lobha, above.
12. Styana, Laxity or Lethargy
Gloominess; inability to focus; withdrawal. This is something like depression, perhaps.
13. Thina, Sloth or Torpor
Also dullness of mind or sluggishness.
15. Vicikitsa, Doubt
This is a doubt that leads to indecision. The doubter may be of two minds regarding doctrine, particularly about the Four Noble Truths, and this leads to vacillation in practice.
[This is an article I wrote for the Buddhism section of About.com. However, since About.com has removed it from their servers, all rights revert to me.]