Software products are an everyday phenomenon. Yet, there are very few scientific studies reported on the engineering of software as a product in particular. This paper stipulates the urgent need for more research on product software. The various terms used for software products are reviewed and some categorizations of software products are presented. Moreover, we provide a software product development framework to position the key business domains in a product software company. From the perspective of the software product development framework, we evaluate the related work and provide directions for future research.