Deontology is the study of moral obligation. It deals with what a person ought to do. "Ought" implies a moral necessity, and therefore a duty an individual should perform. Deontological ethics deals not with the result of actions, but the nature of actions. Take for example someone who does something right because he is forced to by social pressure. Has he done something truly good? The answer would seem to be "No," since his motives as a standard for what is right were improper. Correct moral actions are the result of correct moral duties.
Deontology asks, why should someone behave or believe in a certain manner? Why is one thing right and another thing wrong? Is there something ontologically necessary with "rightness" and "wrongness"?