Originally, when dinosaurs were first discovered, paleontologists assumed that because dinosaurs were reptiles they must be cold-blooded (ectothermic). Cold-blooded is not a strictly accurate term, because reptiles do not really have "cold" blood. Instead, it means that they must rely on the temperature of their environment to regulate their body temperature. Reptiles do this by basking in sunlight to warm up, and by retreating to shade in order to cool down.
The extinct mammal-like reptile Dimetrodon is believed to have used its sail to assist in basking.
Mammals are considered to be "warm-blooded" or endothermic. This means that they are able to regulate their body temperatures internally, without relying on the external environment. Of course, we all know that standing in the sun on a chilly day, or in the shade on a hot day can help you feel more comfortable. But we humans, as well as other mammals, are able to keep our body temperatures constant without being dependent on the sun or shade to cool us up or warm us down. This ability to regulate temperature is also sometimes known as "homeothermy," meaning "same temperature."
But even among modern reptiles, warm- (endothermic) vs. cold- (ectothermic) bloodedness is not entirely black and white. Larger reptiles like crocodiles and alligators do a pretty good job at keeping their body temperature fairly constant, although not within the same narrow target range that we see in mammals, nor do they have body temperatures as high as mammals. Basking certainly still helps in warming them up for action.
Birds, our only living descendants of dinosaurs, are "poikilothermic," which means that their body temperature does vary with the external environment, but they regulate it by means other than basking. Many birds lower their body temperatures and metabolisms at night to conserve energy.
Mammals that are capable of hibernating or estivating can also lower their body temperatures so they are able to conserve energy that would otherwise be used in maintaining a warmer body temperature.
Currently, most paleontologists believe that dinosaurs were somewhere in between warm- and cold-blooded. Dr. Jack Horner at the Museum of the Rockies, Montana State University has been doing a lot of work with the bone histology (cell structure) of mammals, dinosaurs, birds, and reptiles. He has found that dinosaur bones have a lot of blood vessels running through them, similar to birds and in contrast to very few blood vessels in reptiles. This would indicate that dinosaurs had metabolisms that were higher than is seen in modern reptiles, and more similar to modern birds.
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