Rooflights, skylights, sun tubes/pipes, glass roofs and roof lanterns. They are all ways of bringing in daylight, sunlight and ventilation into buildings. They are necessary to provide natural light and ventilation rooms that do not have outside walls but are equally useful to bring a unique quality of light. A surprising amount of drama can be achieved with quite modest top lighting.
A basic velux rooflight can be installed into a pitched roof with boxing around it to create a form of light well or tunnel. The flat ceiling iss retained and the rooflight brings in lots of daylight into an otherwise drab interior. A more dramatic way of incorporating rooflights into a space is by opening up the ceiling and vaulting the space so that the ceiling is sloped. This is obviously only possible on upper storeys or single storey extensions.
A whole range of sizes are available from very small sizes that fit within existing rafter spaces right up to huge 2m units. These will need significant structural support owing to their weight especially if an opening type. Rooflights can generally be installed under Permitted Development rights, provided they meet certain criteria and the building is not Listed or on designated land (Conservation Area, National Park, etc) but you should make checks prior to installation.
Some rooflights are possible to install flush with a slate or tiled roof covering. These are generally preferred for conservation projects and Listed Buildings as the rooflines are less disturbed. They look better on all projects, but tend to cost a little more.