Hypocritical involves acting in a way that goes against your stated beliefs. If you drive drunk despite the fact that you are the president of Students Against Drunk Driving, you are truly hypocritical.
Hypocritical comes from the Greek hypokritikos which means acting a part. The word comes up most often when discussing political and religious figures who are sometimes caught engaging in behavior that goes against their professed beliefs. A person who engages in hypocritical behavior could be called a hypocrite, their behavior, an act of hypocrisy.