Cold-blooded Reptiles and Amphibians
Amphibians became the first vertebrates to live on land, and like their "cold-blooded" reptile relatives, depend on external energy sources (such as the sun) to maintain their body temperatures. This, combined with their slow metabolism, make reptile and amphibian lifestyles and abilities dramatically different from their mammalian counterparts.
Mammals (and birds) are endotherms, which means they generate heat internally, through a high metabolic rate, and usually maintain a fairly high and constant body temperature regardless of ambient temperature (within reasonable limits); this is often called being "warm-blooded."
Reptiles, in contrast, are ectotherms. They obtain heat from outside sources, like the sun, and regulate their temperature through behaviors such as basking or seeking shade. Their body temperature is roughly the same as the ambient temperature.
Turtle basking to raise its body temperature