Even though Social Media is a very used term these days, its definition varies among the social media experts.
According to Wikipedia social media are the media for social interaction, using highly accessible and scalable publishing techniques. Social media use web-based technologies to turn communication into interactive dialogues.
The term social media is also related to Web 2.0 philosophy. This philosophy change the way business communication has been performed until now. Communication now is a two – way road, where people can respond any message given by any company regardless of their size. This characteristic gives power to any person and even more when the response can be conveyed at the speed of light all over the world at a cost close to zero!
These three characteristics (two way-communication, speed and cost) once incorporated into the traditional media; open your brand and company to the scrutiny of current and potential customers, and that could lead to more changes in the way your company has
been doing business.
Classification of Social Media is also difficult and varies according to experts. More over when originally very different platform are now integrating one another. Even though this obstacle I found a very interesting and academic classification given by Catherine Parker:
Blogging (Blogger, WordPress, TypePad)
Micro blogging (Twitter, FriendFeed)
Social Networking (Facebook, MySpace)
Social Bookmarking (Digg, StumbleUpon, Delicious)
Multimedia (YouTube, SlideShare, Flickr)
Reviews and Opinions (Yahoo! Answers, Epinions, eHow)
Wikis (Wikipedia, Wikia)
What every author on social media seems to agree is that social media is only new set of tools to be used in your marketing campaign.
If you want to get more into Social Media I recommend you a few books:
Lon Safko. The Social Media Bible. New Jersey, USA. John Wiley & Son, 2010.
Catherine parker. 301 Ways to Use Social Media to Boost your Marketing. San Francisco, USA. McGraw Hill, 2010.
Kelly Mooney and Nita Rollins. The Open Brand: When push comes to pull in a web-made world. Berkeley, USA. New Riders, 2008.
Tom Funk. Web 2.0 and Beyond. USA.Praeger Publishers, 2009.
Omar

