The primary difference between an endotherm and an ectotherm lies in thermoregulation. Endotherms, often referred to as warm-blooded animals, can maintain a stable internal body temperature regardless of external environmental conditions through metabolic processes. Examples include mammals and birds. In contrast, ectotherms, or cold-blooded animals, rely on external sources of heat for temperature regulation, leading to fluctuating body temperatures that can impact their behavior and activity levels. Examples of ectotherms include reptiles and amphibians.