A GDB (Graph Database) is a structure used for storing and recalling the state of the Graph Screen without storing the screen pixel-by-pixel. This means it is usually smaller than a Picture, which does save the screen pixel-by-pixel.
What a GDB stores is dependent on the current graphing mode (of the following: Func, Param, Polar, Seq). It will store all variables relevant to the current type, but none irrelevant to the current types. If the current mode is Func, a GDB will store Y= function variables, but not Polar functions for example. If the mode is not Func, Xres will not be stored.