In the last few years, people are increasingly demanding personalized information to carry out their daily activities. Information systems are needed to manage a representation of the user's situation, identify user needs and preferences, and implement information retrieval techniques that pull together data from diverse and heterogeneous sources. It is necessary to define and formalize context models for achieving these goals. In this paper, we present a formal context model based on advances on the Semantic Web. The model is compounded by four independent and related ontologies: users, devices, environment and services. Each of these ontologies describes general concepts and relationships involved in intelligent environments. The proposed design enables model specializations to particular domains and interoperability with external ontologies. Moreover, the model supports inference mechanisms to enhance the automatic context generation and the proactive behavior of particular services. Finally, this paper shows a specific prototype that offers personalized and context-aware information to the user, aided by the context model.