In the recent years, the diversification of wireless sensor network (WSN) applications has made the design space of such networks very large. This poses numerous challenges to researchers and increasingly great difficulties for designers to make decisions regarding proper technical solutions. As a result, before any design implementation, it is essential that wireless sensor networks be subject to rigorous analysis. This paper provides an original contribution to this analysis by classify-ing wireless sensor network applications with respect to their functional properties. Five distinct general classes of wireless sensor networks are thus derived: body area sensor networks, data collection networks, location-sensing networks, multimedia sensor networks and control-oriented sensor networks. For each class, we give illustrative applications, discuss in detail specific features and characteristics, and describe general architectural functionalities and some of their technical implications such as power consumption, network topology, data delivery model and quality of service. This classification would benefit the WSN research and industrial community and lays a groundwork for further study in the WSN area.