Thermoregulation is the ability of an organism to maintain its body temperature within a certain range even when the ambient temperature is very different from these limits.
An endothermic being is an animal that regulates the body temperature, in a typical manner, keeping it at a constant level.
An ectothermic animal is an organism in which the physiological internal heat sources are relatively small or too insignificant to control their body temperature. Thus, these bodies depend from heat sources from the external environment. Examples of ectotherms are frogs, toads and turtles.
Animals who are able to maintain a relatively constant body temperature despite large variations in outdoor temperature are called homeothermic.
Animals in which the body temperature varies according to the environment are called poikilothermic. Examples of poikilotherms animals include invertebrates and aquatic animals like fishes and amphibians.