Virtual Laboratories are an indispensable space for developing practical activities in a Virtual Environment. In the field of Computer and Software Enginering diferent types of practical activities have to be performed in order to obtain basic competences which are imposible to achieve by other means. This paper specifies an ontology for a general virtual laboratory. The proposed ontology provides a mechanism to select the best resources neded in a Virtual Laboratory once a specific practical activity has ben defined and the main competences that students have to achieve in the learning proces have ben fixed. Furthermore, the proposed ontology can be used to develop an automatic and wizard tol that creates a Modle Clasrom using the practical activity specification and the related competences.