Hypocrisy is the act of professing a belief, emotion, or moral value, while acting in a way inconsistent with those statements. People are usually charged with hypocrisy when they either insist that others do something that they themselves do not do, or else condemn others for acting the same way as they themselves do.
Often what is most frustrating about hypocrisy is not the actions themselves, but the fact that the hypocrite refuses to acknowledge any contradiction between their words and actions, or may even try to hide certain actions. This is a blatant form of intellectual dishonesty.
There are many possible causes for hypocrisy. A hypocrite may be an outright fraud, promoting a cause he or she doesn't at all believe in. A hypocrite may be experiencing cognitive dissonance, or somehow try to rationalize an action by giving an excuse ("It's OK when I do it, because...").
In some cases, a hypocrite who commits a "sin" may be unable to stop doing it and cleanse their conscience, because of addiction or coercion, because they just enjoy it too much, or because it is a one-time error that is too terrible to forget about. In those cases, unable to escape their feelings of guilt, the hypocrite may condemn that sin even more strongly in public, as a way of atoning for it.