Social media is such a hot word these days that everyone and their mom is using it. In spite of its popularity, though, not everyone knows exactly what it means.
Social media is simply a web site that has an instant feedback mechanism; that is, it provides users with information, while at the same time allowing users to provide their own information. This is in contrast with regular media wherein the information is usually a one-way affair.
Social media can be further broken down into social networking (Facebook), social news (Digg), social bookmarking (Delicious), social photo/video (YouTube, Flickr), and wikis (Wikipedia).