Tree Peonies are peonies which are deciduous.  The above ground growth is perennial in nature and in the spring the plant resumes growth from the point which growth stopped the past fall.  Tree peonies require a bit more care tahn herbaceous peonies because their buds can get nipped by spring frost. Boreas is an example of a tree peony.

Itoh Peonies or Intersectional Peonies are crosses between the herbaceous and tree peonies.  These peonies die back to the ground in the fall which means that the new buds in the spring are less likely to be killed by frost than tree peonies.  Itohs have the same beautiful flowers that tree peonies have. Bartzella is an example of an intersectional peony.