Social Media is a ton of elements of the internet. Breaking it down into classes it goes:
Collaborative Projects : Wikis and the biggest Wikipedia.com
Blogs and Microblogs : WordPress and Twitter
Video Communities : Youtube and Dailymotion
Social Networking Sites : Facebook and Myspace
Social Bookmarking Sites : Stumbleupon, reddit and Digg
Visual Communities: Pinterest and Instagram
For Marketing we make use of Blogs, Videos, Social Bookmarking and Social Networking Sites.