One major division that can be used to classify animals is their heat source. You have heard the terms warm blooded and cold blooded. The scientific terms are endotherms and ectotherms. Endotherms are warm blooded organisms. They derive heat to survive from their internal bodies, namely from metabolic processes and chemical reactions. Mammals are a perfect example of endotherms. They can live in cold and frigid regions of the planet because they have adapted layers of sub-dermal fat and external coverings like fur or feathers to trap their body heat.
Ectotherms are cold blooded organisms. Their internal body chemistry produces only a negligible amount of heat, so they have to expose themselves to warm environments to maintain their body temperatures. Reptiles are ectotherms, that is why they can often be found basking in the sunlight on sunny days. As ectotherms, they are not found in consistently cold areas since they would not be able to maintain their proper body temperatures due to heat loss to the surroundings.