Basic Freemasonry consists of the three 'Craft' degrees (Entered Apprentice, Fellow Craft and Master Mason) completed by the Royal Arch degree (Chapter).
There are many other Masonic degrees and Orders which are called 'additional' because they add to the basis of the Craft and Royal Arch. They are not basic to Freemasonry but add to it by further expounding and illustrating the principles stated in the Craft and Royal Arch. Some of these additional degrees are numerically superior to the third degree but this does not affect the fact that they are additional to and not in anyway superior to or higher than the Craft. The ranks that these additional degrees carry have no standing with the Craft or Royal Arch.
from the UGLE web site