The all-knowing Wikipedia defines social media as "Internet-based tools for sharing and discussing information among human beings." But really, social media is about connecting, interacting, and sharing online.
From a promotional and marketing perspective, things are not so clear cut or easy. The biggest problem is that some people think of social media as just one more channel to push sales. You can but you have to do it gently and with your customers interests first. This is not a position for an intern.
SEO - Social Media Search - Real-Time Search - Online Marketing
I snagged the above graphic form readwriteweb. They have a great post about, Google's New Social Search Is A Big Chess Move Against Facebook.