Software Product Line is a program development methodology, which is a subset of product design methodologies called Product Family Engineering, and Product Line Engineering. It is a relatively new method for creating new products.
Product family/line engineering is a means to create an underlying architecture of a product platform, usually by some organization. The architecture is based on commonality and similarity. Product variants can be derived from the basic product family, creating opportunities to reuse and differentiate based on products in the family. It focuses on the process of engineering new products in ways that make it possible to reuse product components and apply variability with reduced costs and time. It is about maximally reusing components and structures.