This document was prepared to acquaint users of the FGDIS data base (and to a lesser extent, EPA utility FGD survey report recipients) with the major equipment and component classification used in the system. In addition, it contains such things as the problem and solution codes used in conjuntion with the FGD system performance data, as well as definitions of common terminology used to describe FGD systems and power plants in general. Along with the classifications of the various complex equipment items and components that comprise FGD systems, brief functional descriptions and illustrations are provided. This is intended to provide FGDIS system users with a basic understanding of these major equipment items.