The document discusses the product life cycle concept which outlines the different phases a product goes through. It describes the stages as research and development, introduction, growth, maturity, and decline or rejuvenation/termination. The concept is used to forecast sales behavior, assist with marketing strategy formulation, and analyze deviations from projections. The stages involve complex research initially that absorbs significant resources before sales are achieved in the introduction phase through promotional activities. Growth sees increased distribution, promotions, and product improvements for the longest and most profitable phase.