to quote wikipedia:
Software Configuration Management (SCM) is part of configuration management (CM). Roger Pressman (in his book) Software Engineering: A Practitioner's Approach, says that software configuration management (SCM) is a "set of activities designed to control change by identifying the work products that are likely to change, establishing relationships among them, defining mechanisms for managing different versions of these work products, controlling the changes imposed, and auditing and reporting on the changes made." In other words, SCM is a methodology to control and manage a software development project.

SCM concerns itself with answering the question: somebody did something, how can one reproduce it? Often the problem involves not reproducing "it" identically, but with controlled, incremental changes. Answering the question will thus become a matter of comparing different results and of analysing their differences. Traditional CM typically focused on controlled creation of relatively simple products. Nowadays, implementers of SCM face the challenge of dealing with relatively minor increments under their own control, in the context of the complex system being developed.