We categorize and compare different representations of program dependence graphs, including the Control Flow Graph (CFG) which is a sequential representation lacking data dependences, the Program Dependence Graph (PDG) which is a parallel representation of a sequential program and is comprised of control and data dependences, and more generally, the Parallel Program Graph (PPG) which is a parallel representation of sequential and (inherently) parallel programs and is comprised of parallel control flow edges and synchronization edges. PPGs are classified according to their graphical structure and properties related to deadlock detection and serializability.