Tags assigned by users are considered to have potential in categorizing web resources despite the fact that they do not use a controlled vocabulary and are contributed by non-experts. This study explores the potential of using tags to replace controlled vocabulary as a method of organizing web resources. Author-assigned keywords and users-assigned tags for a shared data collection are compared. The data was gathered from scientific papers that have both author-assigned keywords and user-assigned tags. Author-assigned keywords and user-assigned tags were compared using similarity measurements, classification systems, and clustering methods.