A roof window is a specific unit designed for an installation that's within reach. VELUX's roof window sash can be pivoted so you can clean the exterior pane of the glass from inside. Additionally, some larger roof window models meet building code requirements for egress (emergency escape).
Often, roof windows are installed in attics that are used as living spaces. Skylights, on the other hand, are usually installed in "out-of-reach" applications.