Today the terms roof window and skylight are used interchangeably; neither is a clear indication of any certain characteristics or special features.
There are two types of roof windows / skylights: fixed, non-functional units that remain closed and vented, functional units that can be opened. Originally the term "skylight" applied only to fixed, non-functional windows that were installed only for the purpose of allowing daylight to enter the building from above. And the original use of the term "roof window" was to indicate that the unit had a small ventilation flap or was capable of opening to some degree. Within the last 20 years, it is common to see both roof windows and skylights explained as being either the fixed or venting type of window.
Choosing between those two features, along with your roof type and installation method, should help you determine which product you need, regardless of what you call it.