Basically you have two choices when it comes to skylights.
Fixed Skylights are skylights that do not open. Their purpose is to allow natural light in. They are generally more efficient because they can be more airtight and they offer the most in terms of safety and security.
Operable Skylights are skylights that open. They provide fresh air ventilation when desired. They can be used to create stack effect ventilation, which draws cool air in lower windows and allows warmer air, which rises, to escape through the roof.
While there are many different types of operable windows, when it comes to operable skylights, only the awning-type, or ones that swing out from the bottom, are suitable. They usually operated with either a roto-gear and crank or a simpler push-out lever. The hardware does not allow them to open all the way.