Program slicing transforms a large program into a smaller one that contains only statements relevant to the computation of a given function. It has been shown that program slicing can be useful in program understanding. Traditionally, program slices are represented in the textual form. Although slicing does narrow the size of the program, the textual representation of a slice does not provide much guidance in the understanding of large programs. In this paper we present program slicing concepts on the module level that allow for better understanding of program slices of large programs and their executions. These concepts have been developed for static and dynamic program slicing and are combined with different methods of visualization to guide programmers in the process of program understanding. The presented concepts have been implemented in the slicing tool that is used to investigate the usefulness of these concepts in the process of understanding of large programs.