Reptiles and amphibians are ectothermic (thats cold blooded to you and me)
The term cold blooded is something of a contradiction in terms however, it is more accurate to say that reptiles must absorb temperature from their environment to stay warm,having no direct control of their own blood temperature beyond seeking external sources of heat(except the varanids - the monitor lizards, who have been proven to be able to chemically raise their internal temperature by up to 2 degrees c) but odd as it may sound, many reptiles maintain a higher average blood temperature than we do. It is not actually normal for most tropical or desert reptiles to be cold, either at blood temperature or to the touch. Most typical species will have a blood temperature after basking of between 80 and 95f, which is not so far behind the human 98.4f !
Needless to say, this means almost all pet reptiles will need supplementary heating, in the UK's reliably lousy climate. Not only that but on particularly hot days you may also have to provide them with a cooler retreat, and this brings us on to the subject of a thermal gradient.
For reptiles you dont provide one flat temperature. Not only do temperature preferences vary from species to species, but also according to size, breeding condition, and what mood the animal is in, what activity it chooses to engage in,if the animal is diurnal or nocturnal, how and what is has been eating, and whether its awake or sleeping.
So how do you know what temperature to provide? Basically you cant know, its impossible to predict what temperature a reptile needs minute by minute,so you provide an accessable range of temperatures all the time, and it is the reptile itself that chooses whether to go to heat or retreat from it , or just hang about in the midranges, quite simply by moving themselves around.
This of course means that reptile vivaria have to be of a certain size. Big enough to have a heat source, and a cool area, and a midpoint area of some size that is usually in the average midpoint of temperatures. Generally a good thermal gradient is very difficult to achieve in less than 3 feet of space, and for most reptiles a vivaria with space equivalent to a 30 gallon tank is really the minimum required space to achieve an effective thermal gradient, though obviously some creatures need a great deal more space than this. Of course given you want as much range in temperature as possible in a small space,so you will want the main heating and spotlights at one end, and some cover or cave at the far end , perhaps kept a little moist with a light spray of water once in a while to keep it cool.
For example, say we have a green anole (anolis carolinesis),its an appropriately sized animal for a 30 gallon capacity vivarium, a tropical species that appreciates average tropical temperatures much as green iguanas or thai water dragons do. Now for this species it needs a basking spot area, usually under a spotlight or ceramic heater, of around 90-95f. This is the area it will use to warm itself daily. It cant stay there forever, because if it does, it will become too hot and die.So having warmed up for a few minutes it will then stay in the main area of the vivarium which should be around 79-86f, and there it will stay doing its daily things, occasionally warming up under the light, or heading off to the opposite cool corner that should be around 75 f or a little less.
So in no time at all , youve realised that your lizard needs three discrete temperature areas to live thrive and survive. A hot spot, a warm main area, and a cool retreat. Not one, not two, it needs all three.
At night the lights must go off for the lizard, being diurnal as it is in this case to get a good nights rest, and this is when you turn the spotlight and any striplights off and allow the entire vivarium to cool to arounf 75-78f. This is telling you something about the equipment you need. The tank temperature must be controlled, not by the lights, but either by heating the room appropriately, or in cases where this isnt possible, by using something like a thermostatically controlled ceramic heater that emits no light.
Now there are those who make do without such equipment, but having been one of the very few in this country to see a green iguana through from 18 months to a grand old age of 21 years, I can assure you this kind of thermal control is essential for the longevity and health of reptiles.Granted you can make exceptions for noctural creatures like many of the geckos and some snakes, who by virtue of nocturnal living rarely bask, but generally the diurnal creatures need this kind of control and will fare poorly without it. Diurnal reptiles are, broadly speaking, more difficult to keep than nocturnal ones.They have an affinity with light, associate heat with light , and light with wakefulness. Lights for photoperiod and UVB and UVA exposure for vitamin d3 sythesis should as much as possible be seperate pieces of equipment from heating apparatus.There will be times when you will wish to cool a vivarium or raise its temperature, and it really does help if you are not relying on lights for the primary heating of a vivarium. Diurnal reptiles will always however , choose to sit under a bright light source than a dark warm heat source , most of them instictively associate light with heat, and wont change this opinion no matter what you do. You can for example sabotage your reptile by only having a striplight and a ceramic heater that are distanced apart. The reptile will always choose the light, even if its cooler than the ceramic heater. Only reptiles with heat sensing pits such as the pit vipers will overcome this.Heat and light completely regulates reptilian behaviour, and consequently since their behaviour is so linked to their temperature, their digestion, and immune system, they must be cared for with some accuracy. Not to do so risks digestive issues and consequently much increases the likelihood of metabolic bone disease. If the animal isnt warm, the calcium it eats, wont be getting digested and added to its bones even if you have uvb lights and use d3 supplments.
Nocturnal creatures too will bask given the opportunity, but many will content themselves with non light emitting heaters, infra-red lamps, mats, and strips, being as they are used to absorbing heat ventrally from surfaces still warm from the sun after nightfall. A few of the tougher species can get by most on a decent ambient temperature, though its always nice to give them a warm area to enjoy.
Tortoises too must be cared for under similar conditions to diurnal lizards, most freshwater turtles also require similar principles , but because of their semi aquatic nature, they tend to be a little more flexible about their temperatures, but they should still be getting basking lights and some ambient heat, the water too often needs to be managed for temperatures, and since they come from the same rivers as many fish we keep in the hobby, how to provide water heating should be no mystery.
Diurnal reptiles are all designed to take heat from above , namely the sun, and all of them have vascularised tissue on their upper skin surfaces designed to transmit heat to the blood as efficiently as possible. Even the scutes of a turtle or tortoise are designed in this way. Most do not posess similar vascularisation on their bellies, and this is why I will be quite vehement in nver recommending infra red mats, hot rocks or similar devices that provide heat from below as acceptable for diurnal reptiles. Thermal burns and deaths are common, and often such devices fail and overheat when larger reptiles sit upon them.
Other major problems are to do with ventilation. Reptile keepers often make huge errors in overdesigning a vivarium not to loose heat. Ventilation has to be provided. In converted aquaria its useful to drill vents in a couple of inches above floor level, and least partial mesh screen tops. Standard aquarium lids will not do. In theory most vivaria should have at least two vents, one high, one low, and a constant steady draft powered by rising heat from the equipment and lights creates a constant flow of fresh air, The principle is called convection ventilation. It will keep reptiles from overheating, and filling with co2, and will prevent fungal outbreaks. You do have to compensate by spending a certain amount of electricity on heating and sometimes you will have to spray some species with water occassionally, but that is all standard stuff in the herpetological hobby. Most reptile cages also involve some water, at the very least a bowl or a damp substrate, so you dont want anything electrical at ground level anyway. Feeder crickets and mealworms also tend to gnaw on cables, and reptiles move around just about anything.
Amphibia and various insects and arachnids are perfectly happy with blanket temperatures, but they too need ventilation. There are few species of treefrog that will bask and will respond to light rather than heat like most of the diurnal reptiles, but most are happy in average tropical temperatures of 75-86 f.
Hope that helps the newbies on the basics of heating.