By definition, perennials are herbaceous, or fleshy, plants that grow, bloom and usually die back to the ground in the winter with the roots surviving cold tempretures, then they send up fresh growth and flowers each spring, repeating the cycle every year.  Some perennials will survive for many years and others last just a few seasons.  Some develop woody stems and lose their leaves but do not die back, while others retain their leaves during winter.  There are tens of thousands of different perennials all over the world.