Virtual Laboratories are an indispensable space for developing practical activities in a Virtual Environment. In the field of Computer and Software Engineering different types of practical activities have to be performed in order to obtain basic competences which are impossible 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 needed in a Virtual Laboratory once a specific practical activity has been defined and the main competences that students have to achieve in the learning process have been fixed. Furthermore, the proposed ontology can be used to develop an automatic and wizard tool that creates a Moodle Classroom using the practical activity specification and the related competences.