A large number of Web services are already available on the Web, and the type and number of Web services grow on a daily basis. As the Web service paradigm becomes popular and more and more applications are developed or deployed as Web services, the need for defining different matches of Web services becomes manifest. This has raised the interesting research point for defining Web service matches. In this paper, we concentrate on the vital part of Web service matching - the capability matching and formally analyse and define exact match and plug-in match of Web service capabilities using abstract finite-state machine approaches. Based on the above work, we propose a new lightweight capability description language of Web service (SCDL) and explore the major stages of service capability matching. Then we focus on plug-in match and formally define it using formal logic methods. Moreover, we provide some feasible approaches and solutions for comparing two service capabilities. The work presented in this paper lays down a solid foundation for the framework for tackling the challenges of Web service discovery and composition on which we are working.