See image above. Effectively, this means that if your mouth is full of food you can still breathe, but a reptile has to breathe around its food. Not all mammals choose the same temperature, and some are not very good at it. Bats, for instance, tend to cool down considerably when at rest, this is partly because of the large surface area for heat loss supplied by their wings. Finally there are the Naked Mole Rats, Heterocephalus glaber which have lost this ability, probably because they do not need it anymore.
Birds are also endothermic.
So now you have a better idea of what a mammal is. Ultimately though, mammals are part of the beauty and diversity of life that makes this world so good to live in. Unfortunately, many people in positions of power fail to appreciate this beauty and the precious fragility of life and many mammal species are currently in danger of disappearing for ever. So far 86 species and 24 subspecies, a total of 110 distinct taxa of mammals have already become extinct since mankind started keeping records. Apart from these 241 taxa of mammal are currently on the IUCN critically endangered list.