Different species have slightly different temperature and light positioning preferences, but all of them need the same thing. Please read this general page and then refer to your animal's specific care sheet.
All chelonians require UVA/UVB lighting. UVA and UVB are a specific type of rays that these animals are exposed to when basking in the sun in the wild. We humans cannot see it, but it is essential to them. There is also a third type referred to as UVC which they do not need much of. I will go into detail about each one here.
Many turtles and tortoises use UVA for vision. It's the difference between daylight and nighttime for them, and in nature they use it to know, in effect, what time of day it is and whether they should be out basking or holed up in their burrow. Many basking spot bulbs and all UV lamps have some level of UVA, so unless your bulb specifically says it's for nighttime use do NOT use it as a 24/7 hour heat source because it will mess up their "body clock" and interfere with sleeping.
When turtles or tortoises bask in the sun and are exposed to UVB, their bodies make its own Vitamin D3. This vitamin is essential to drive calcium into the bones and make them hard and strong. If they aren't getting enough Vitamin D3 but do have enough calcium, their health will deteriorate the same as if they were eating no calcium because their body is not able to process it. Your shelled friend can get enough Vitamin D3 through its diet, so for very temporary setups you can do without this. However, it is impossible to know if they are getting the right amount before irreversible damage is done, and I am a strong believer in simulating nature. The sun shines over all parts of the world, so your turtle or tortoise should have access to it be it real sun in the outdoors or an artificial UVB lamp shining into their indoors enclosure.
UVC is a third type of UV rays, but it is mostly good for plants. For aquatic turtles, it is sometimes recommended that you make use of a UVC filter to reduce problems with algae growth and shell rot. I have never used one and am experiencing no problems, so I would recommend you check water quality first. If water quality is causing these problems, it could affect your turtle too, even with the filter in place! However, some turtles are particularly sensitive to shell rot--soft-shelled turtles, for example--and this may be your solution, second to keeping the PH at the correct level.
When fitting an indoors enclosure with UV lighting, it is important to remember that the sun always shines from above. A turtle or tortoise's head is therefore shaped accordingly because UV light shining directly into the eyes can damage them but the shape of the head will block this if the light shines from above. Therefore, you should always position UV bulbs so that they shine straight down into the tank, not at an angle as would occur with a clamp lamp. It is also important to follow manufacturers' directions on how far away or close to the basking area the bulbs should be placed because of safety concerns. Also always put it into a reflector to ensure that all UVB rays go down into the enclosure and to protect your own eyes when you go to check on your beloved pet!
Turtles and tortoises always absorb heat from above when basking in the sun. Therefore, you should try to use a heat-emitting lamp on the basking area in the same place as your UVB lamp shines. Never use heat mats, heat rocks, or any kind of undertank heater! These are only suitable for nocturnal reptiles, such as leopard geckoes, who always absorb their heat from the ground and rocks. Turtles and tortoises are less sensitive in the belly area and may get too hot before they realize it and get burns if an undertank heater is used.
Mercury vapor bulbs emit heat as well as UVA and UVB, so one of these may be sufficient. If you choose to use a tube bulb for UVA/UVB or if additional heating is required to achieve the desired temperatures, a basking spot bulb is acceptable for daytime heat. Arid-land tortoises do not need a heat source during the night unless it gets extremely cold, or they may become reluctant to eat. This is because in their natural habitat it does get cold at night and they feed in the morning after warming up again. If a nighttime heat source is required, a ceramic heat emitter, red incandescent bulb, or an infrared incandescent bulb are all suitable. Reptiles cannot see red light as emitted from the red incandescent bulb, and the other two options give off no light.
It may be beneficial to have a heater for the water in your aquatic turtle's tank. They do have a preferred temperature range, so if the room they are kept in is below that range a heater is good. It's also necessary to keep outdoors ponds from freezing over if you decide to hibernate your aquatic turtles outdoors.
Now I'm going to add a tip. If it happens that your power goes out and you cannot get your pets to a warmer location that has electricity, you can keep them warm with hot water bottles. This would be if you have a gas hot water heater or a gas stove, of course... If you are unable to do anything and temps do not drop below 40_F, they should be fine without heat.
Well, that about covers all your pet's heating and lighting needs. In my experience, second to getting a good enclosure, this is the most expensive part of chelonian ownership, but it is one you absolutely cannot do without.
