A load line is used in graphic analysis of circuits, representing the constraint other parts of the circuit place on a non-linear device, like a diode or transistor. A load line represents the response of a linear circuit to which the nonlinear device in question is connected to. The operating point is where the parameters of the nonlinear device and the parameters of the linear circuit match according to how they are connected while still adhering to their internal systems.
From Wikipedia
Peccavi, March 2009
