An ectotherm is an animal in which the internal body temperature is the same as the temperature of its surroundings. Fish, amphibians, and reptiles are ectotherms. Ectotherms can not control their internal body temperature and therefore have to stay in either the sun to stay warm, or the shade to keep cool.

Endotherms are animals that can maintain a constant internal body temperature, regardless of the temperature of their surroundings. Birds and mammals are endotherms. Do you have a constant internal body temperature? You bet! You are also an endotherm. Humans have a constant body temperature of 98.6 °F.