A herbaceous perennial dies back to the ground in winter and then re-grows the following spring. A hardy perennial will still have top growth in winter.

Thanks Moongrower! I wondered if this was the case but have never been sure of the difference.

a perennial is a plant that lives for many years. Hardy tends to mean frost hardy often down to -5 or lower depending on the species. herbaceous as mg says dies down totally and regrows from the base every year. Then there are perenials like bulbs tubers etc. as well as shrubs and trees. so a hardy perennial is a subset of the large group of plants.

Thank you Seaburngirl for the further clarification!