Self discipline is the assertion of will power over more base desires, and is usually understood to be synonymous with self-control. Self-discipline is to some extent a substitute for Motivation. Discipline is when one uses reason to determine the best course of action that opposes one's desires, which is the opposite of fun.
Virtuous behavior can be described as when one's values are aligned with one's aims: to do what one knows is best and to do it gladly. Continent behavior, on the other hand, is when one does what one knows is best, but must do it by opposing one's motivations. Moving from continent to virtuous behavior requires training and some self-discipline.
Adapted from Wikipedia