"Warm-blooded" is used to describe animals that generate their own heat. Warm-blooded creatures are often called ectotherms. Mammals and birds are warm-blooded; reptiles and amphibians are cold-blooded.
Reptiles and other cold-blooded animals must draw heat form the environment, and they are slower and less active when the temperature drops. They also have a slower metabolism, which is why they often spend long periods of time resting. However, being cold-blooded has an advantage: they do not have to eat as much as their warm-blooded counterparts.
In exchange for their more robust dietary demands, warm-blooded animals are able to cope with a broader range of environmental conditions, but hotter weather is often a greater threat than cold weather. Warm-blooded creatures are able to move at a faster rate of speed than cold-blooded ones. This is part of the reason why mammals and birds can migrate long distances.
Some creatures, however, are not clearly warm-blooded or cold-blooded. The dinosaurs, in particular, seem to straddle this gap. If they were strictly cold-blooded, they would have been far slower than most experts believe. If they were warm-blooded, their dietary needs would have been more than most think is realistic. While there is still a considerable amount of debate, many experts now believe that dinosaurs were neither truly warm-blooded nor cold-blooded.