As the amount of information through growing Internet has dramatically increased recently, information professions generated metadata has been reached the limits of their role. Based on the value of information depends on the user, user generated metadata is a growing paradigm in the field of information organization today. This paper firstly introduces the concepts of user-generted metadata, and then examines the cases of user annotation and user tagging. Finally we discuss the effects and implications of user-generated metadata on the development of digital libraries.