To investigate the effects of anonymity on user behavior, we conduct an empirical study of the new and controversial social app, Yik Yak. First, we examine how users use the platform, analyzing patterns in posting, popularity of yaks, and vocabulary. As a comparison, we look at posting patterns on Twitter, which has similar limitations on lengths of posts, but is public and global rather than anonymous and local. Upon a sample of 2.9M posts (1.9M yaks and 1M geotagged tweets) from 20 locations across the USA, we find that interactions on Yik Yak are specific to its location limitations and reflect the schedules of its targeted demographic, college students. Second, we test two hypotheses related to anonymity and communication: (i) whether vulgarity usage is more likely to be acceptable, and (ii) whether unique topics emerge in conversations on Yik Yak. We find that posts on Yik Yak are only slightly more likely to contain vulgarities, and we do not find any significant bias in topic distributions on Yik Yak versus on Twitter; however, differences in vocabulary and most discriminative words used suggest the need for further analysis.