The analysis of mobile behavior is elementary for applications such as city planning, optimization of public transport or mobile communications. However, many applications in the mobility domain require a semantic interpretation of movement information. While physical trajectories can readily be recorded using Global Positioning System (GPS) technology, the semantic interpretation of the data is still a great research challenge. However, GPS trajectories can be utilized to facilitate the manual semantic annotation process, and thus render tedious interviews and manual mobility records unnecessary. In order to realize this schema in practice, two requirements are necessary. First, a conceptual annotation model, and second a graphical user interface which takes advantage of the rich information hidden in physical trajectory data. In this paper, we introduce an extensible trajectory annotation model, which is oriented on the notion of episodes and allows a clear separation of semantic and physical trajectory information. We implemented our model and developed a program to support trajectory annotation independent of the recorded location. We present our program architecture and various features that facilitate the annotation process.