The subclass includes humans. They have external ears, most can suckle on a nipple, and have an ankle specialized for power and range of motion. Therians are often classified by their specialized dentition.
Almost all currently extant (not extinct) mammals are therians. The only exceptions are the platypus and the echidnas (spiny anteater), both of which are prototherianmonotremes.
Bibliography
Vaughan, Terry A., James M. Ryan, and Nicholas J. Czaplewski. 2000. Mammalogy: Fourth Edition. Saunders College Publishing, 565 pp. ISBN 0-03-025034-X