It depends on the convention, however. At some conventions (Boskone, for instance), a "Special Guest" is a category of Guest of Honor with equal rank. At others, (Minicon, for instance), "Special Guest" is used in the Worldcon sense to designate a guest of the con who is being singled out for special notice, but who is not one of the GoHs. There is no algorithmic way to find out which is which: You have to ask.
In general, when fans are speaking carefully, a "guest" at an SF convention is always one of a small number of people who are being specially honored by the convention and whose expenses (but never an honorarium) are paid by the con. When the word "guest" is used to describe someone formally, it never refers to the general run of program participants.
None-the-less, conventions occasionally use the term "guest" loosely, especially in publications, to mean all program participants.