Recently, a lot of research have been studied to apply wireless sensor networks to USN applications. The characteristics of USN applications is that they are varied according to the sort of sensors and sensing objects. They includes military applications, water pollusion monitoring, intelligent traffic system, farm, logistics, location position applications, and so on. There is no traditional method to classify these applications. In this paper, we propose a taxonomy scheme to classify USN applications. The criteria is based on 3 properties, the criticalness of services, the mobility of sensing entity, and the mobility of sensed objects, to classify them into 8 groups. We also describe the technical issues and related works in them. Our taxonomy can make USN applications to be easily understood and memorized.