The semantic web relies on ontologies representing domains through their main concepts and the relations between them. Such a domain knowledge is the keystone to represent the semantic contents of web resources and services in metadata associated to them. These metadata then enable us to search for information based on the semantics of web resources rather than their syntactic forms. In this paper we present an ontology-based approach of web querying, using semantic metadata. We propose a query language based on ontologies and we stress its ability to express approximate queries which is of prime importance for efficiently searching the Semantic Web. We present the Corese search engine we have developed to handle RDF(S) metadata.