We will provide a brief introduction to OWL, in fact OWL2, and the underlying Description Logic, clarifying the semantics and providing examples to help the understanding of this admittedly complex formalism. In particular, we will discuss common misunderstandings around OWL and OWL2, explain the open world assumption, inferences, and the functionality of reasoners. We will use the RacerPro reasoner to demonstrate the benefit of using reasoning for query answering over ontologies. Scalability issues with respect to expressive ontologies as well as huge assertional knowledge bases are discussed.