The main goal of my research task was to examine various approaches to space-time metrics that are commonly referred to as pp-waves and to unify the notation and definitions of these objects. In my talk I shall present the audience with some of the definitions and especially highlight the relationships between these various approaches. After settling what the term pp-wave consists of, and what the heuristics for introducing such object are, I shall introduce some of the basic properties, as well as some of the more advanced classical results (Brinkmann and Rosen coordinates transition, Penrose limit).