At Worldcon, "special guest" designates a person being brought specially to the convention by the convention, who is not a Guest of Honor.
Outside of Worldcon, this term is often used for a convention Guest of Honor which does not fit one of the common categories of (Pro) GoH and Fan GoH.
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, "guest" at an SF convention is one of a small number of people whose expenses (but never an honorarium) are paid by the con. When the word "guest" is used to describe someone formally, it practically never refers to the general run of program participants.
