In the context of a lecture, the capacity to readily recognize and synthesize key concepts is crucial for comprehension and overall educational performance. In this paper, we introduce a tool, the Collaborative Lecture Annotation System (CLAS), which has been developed to make the extraction of important information a more collaborative and engaged process. The system relies on semantically constrained annotation, postannotation data amalgamation and transparent display of this amalgamated data. In addition to describing the CLAS, we report on a user experience study aimed at investigating students' perception of the utility of the tool.