The blueprint for a well-rounded defense contains two classes of defenders: rim protectors and perimeter defenders. The rim protector’s duties include guarding their opponents’ big men and blocking shots around the rim. They’re essentially the last line of defense if the defense collapses.
For perimeter defenders, their job is to stop the defense from collapsing in the first place. They do so by stifling ball movement and preventing their match-up from getting easy shots. In order to excel at this, a defender needs to be each of the following: cognizant of their opponent’s offensive tendencies, extremely agile on the perimeter and annoyingly physical to vex the opponent into turning over the ball.