Specifications are a list of requirements, features, constraints, etc. needed by a product for it to perform its task(s) correctly. They are developed by Systems Engineers
… in consultation with either customers and/or marketing. They are used by the Design Engineers (e.g. Electronics Engineers, Mechanical Engineers, Software Engineers, Civil Engineers) to make sure the system will perform as desired. They are used by Test Engineers to develop necessary tests and procedures to verify, troubleshoot, etc. the finished products. In other words specifications are a standardized formal language used to communicate between the various engineering disciplines working on a project/product so that they all agree and have a common understanding of that project/product. (MORE) 
