Hot end - Cool end
Reptiles being "cold Blooded" rely on external heat sources, they require heating to raise their body temperatures so they can become active and to aid their digestion. There is a danger that a reptile being kept in a captive enviroment under artificial heating can become overheated. In the natural enviroment once a reptile reaches its optimum temperature it moves somewhere cooler. If the heating in a vivarium is incorrectly balanced the reptile will have no escape from the heat, preventing it from cooling down. This can be a stressfull and often fatal situation.
This can easily be avoided by making sure your reptiles enclosure has a hot end and a cool end. Always make sure you have a thermometer at either end of the vivarium so you can monitor the temperature gradient, place all heating at one end of the vivarium and when using a thermostat place the probe in between the heater and the cool end of the vivarium. By placing the probe in this location in theory will ensure that the cool side will always be lower than the hot side.
If your pet reptile makes use of branches and plants such as a chameleon or hides and caves such as a leopard gecko or snake make sure you have these available at both ends. For example if a snake is too warm at the hot end and is forced to find refuge in the cold end with out sufficient hiding space it can lead to unnecessary stress.