Most reptiles in the wild bask in sunlight at some point during the days. This is important for their survival. If you are lucky enough to live in a warm climate you may keep your reptiles in outdoor enclosures. Here they will be exposed to natural sunlight and will receive the full spectrum of light needed for health and well being. Most people, though, must keep their reptiles indoors where they are only exposed to our artificial lights which do not provide a full spectrum like the Sun. This is why special bulbs must be purchased for reptile tanks. A mix of artificial lights is used to mimic natural sunlight.
The main light you need to worry about is UV (ultraviolet) light. Humans cannot see this light and reptiles can only see one type. There are three types of UV light: UVA, UVB, and UVC.
UVB: This is the most important light. For this you need to go to a pet shop or reptile supply company and buy a special bulb. All diurnal (active during day) basking reptiles require them. The UVB light penetrates the animals skin causing a chemical reaction that changes pre-vitamin D3 into vitamin D3. without vitamin D3 organs can fail, deformities can occur, and death will eventually happen. Nocturnal reptiles (leopard geckos, crested geckos, tokay geckos etc.) and amphibians (frogs, salamanders, etc) do not need added UVB light. They have adaptations that allow them to make do with less light. When buying a UVB bulb look for linear tube type. They are best.
UVA: Reptiles can see this type of light. They use it to recognize other members of their species and food. UVA light makes reptiles more active and encourages breeding. It also helps with general well-being. Common "full" spectrum household bulbs, basking bulbs, and many UVB bulbs produce it.
UVC: This light is naturally filtered out by earths atmosphere. It is harmful and only used in industry to kill dangerous micro-organisms. You do not need to worry about this. I only mention it because it exists.
Basking reptiles will also need a basking or heat lamp. This simply provides warmth. A simple incandescent bulb can be used for this. It is important for the reptiles metabolism and because the animals body has to be a certain temperature to synthesize vitamin D3. The UVB bulb should be placed near the basking lamp.