Mammals are distinct animals that possess backbones, a constantly warm internal temperature, hairs, or furs on their skins, have 4 chambered hearts, and feed their young with milk. These characteristics however are not all entirely unique to mammals, as other life forms can exhibit some of them.
The constant internal temperature of mammals is also exhibited by birds, and some mammals are even cold-blooded. Not all mammals have four limbs (e.g. whales), all mammals have four-chambered hearts, but this is not specific to just mammals (e.g. some reptiles). Hair and furs are the chief distinguishing characteristic of all mammals.