We propose a conceptual model of software development that encompasses all approaches: traditional or agile, light and heavy, for large and small development efforts. The model identifies both the common aspects in all software development, i.e., elements found in some form or another in each and every software development project (Intent, Product, People, Work, Time, Quality, Risk, Cost, Value), as well as the variable part, i.e., the main factors that cause the very wide variations we can find in the software development world (Size, Age, Criticality, Architecture stability, Business model, Governance, Rate of change, Geographic distribution). We show how the model can be used as an explanatory theory of software development, as a tool for analysis of practices, techniques, processes, as the basis for curriculum design or for software process adoption and improvement, and to support empirical research on software development methods. This model is also proposed as a way to depolarize the debate on agile methods versus the rest-of-the-world: a unified model.