Reddit is a site where people can share links and post comments. Topics are divided into subreddits, and users can also upvote or downvote posts, which can help make a post more or less visible. Jane Friedman has a great post on how authors can market their books on Reddit.
If you’d like to learn even more about Reddit and its potential for marketing, Websitebuilder has created a great infograph that covers pretty much everything you need to know: