Digress (di GRESS) u: to stray from the main subject
Speaking metaphorically, to digress is to leave the main highway in order to travel aimlessly on back roads. When a speaker digresses, he departs from the main topic and tells a story only distantly related to it.
Such a story is called a digression. Sometimes a writer's or speaker's digressions are more interesting than his or her main points.
After a lengthy digression, the lecturer returned to his speech and brought it to a conclusion.