In SOA, composite applications can be developed on the basis of collections of interacting web services. A service's functionality is exposed to the external world by an abstract interface, described by the standard WSDL language, which must be published by service providers to public registries where service consumers can find them. Nowadays, web service discovery has become a real problem, because of the lack of public registries to publish and organize the fairly huge number of existing services. In this paper, we propose an approach based on formal concept analysis (FCA) for classifying and browsing web services. Using this approach, the web services are organized into a lattice structure, to facilitate their browse and selection. A service lattice reveals the invisible relations between the services, enabling the discovery of a needed service as well as the identification of its possible alternatives. Thus, service discovery may be achieved more easily using the service lattice. This facilitates the construction of service compositions and supports them with backup services to ensure a continuous functionality.