Strictly speaking these should be called roof-light conversions, where the windows are in line with the roof, but most installers use Velux windows (because they are the best) and as a result they have become known as Velux conversions.
Generally these are the easiest types of loft conversions and frequently do not require planning permission, as the roof-line is not being altered. We do, however, suggest that you check with your local planning department before going ahead with any structural work.
They can give a dramatic airy interior, as roof-lights are a very effective at bringing light into a space, and for many people this is a cost effective way of gaining more space.
Velux windows providing light from both sides
Velux windows in an Edwardian loft
Awkward roof lines are not a problem