Fifteen yard penalties are given for what kind of offenses?
In both college and the NFL, personal fouls (such as unnecessary
roughness, grabbing the facemask, clipping, roughing the
passer/kicker) and unsportsmanlike conduct.
In college, offensive pass interference is a 15 yard penalty and
defensive pass interference is 15 yards if the foul occurred 15 or
more yards past the line of scrimmage (otherwise, the ball is
placed at the spot of the foul). In the NFL, offensive pass
interference is a 10 yard penalty and, for defensive pass
interference, the ball is placed at the spot of the foul.