Why are we called mammals?
The term mammal is rather recent, and was proposed by Linnaeus in 1758 in the final edition of his book "Systema Naturae." He derived the term mammalia from the words for breast and animal (mamma + animalia = mammalia). From this derived the vernacular English term "mammal."
Basically, members of the class Mammalia are distinguished from other animals by the possession of mammary glands, and hair. However, mammals are distinct in many other ways as well.