As the Web Service paradigm gains popularity for its promise to transform the way business is conducted, the number of deployed Web Services grows with a fast rate. While sophisticated semantic discovery mechanisms have been invented to overcome the UDDI's syntactic discovery solution in order to provide more recallable and precise results, the amount of functionally equivalent Web Services returned is still large. The solution to this problem is the description of the QoS non-functional aspect of Web Services. QoS encompasses the performance of Web Services and can be used as a discriminator factor for refining Web Service advertisement result lists. However, most scientific efforts presented so far are purely syntactic and are not capturing all aspects of QoS-based Web Service description leading to imprecise syntactic discovery mechanisms. This paper presents a novel, rich and extensible ontology-based approach for describing QoS of Web Services that complements OWL-S. It is shown that, by using this approach and by introducing the concept of semantic QoS metric matching, QoS-based syntactic matchmaking and selection algorithms are transformed to semantic ones leading to better results.