This article gathers the results of an investigation to define and to identify models of application of e-learning, its elements and characteristics and the way of measuring impact. It is based on a bibliographical review that analyzes the last contributions, classifies and summarizes them according to diverse criteria. First there is a scientific definition, the context and the evolution. Section two considers three basic approaches of e-learning and is extended by the personal learning environment and the learning objects. It follows then an analysis of the technical requirements for the computerized training. In part four advantages and disadvantages of e-learning are analyzed,and finally a set of indicators is proposed to measure and to evaluate the impact of these systems