Social media is the collective name for social networking sites (e.g. Facebook and Twitter) and other peer-to-peer sharing sites (e.g. Flickr, YouTube and Pinterest).
The core philosophy behind social media is peer-to-peer sharing. Sharing experiences, products, memories, views and opinions. This desire to share can extend to companies' and organisations. The key factor in users of such sites sharing companies' updates is engaging, interesting and useful content. The burden is therefore on companies to produce high value content which users would want to share.
Using social media for marketing
In marketing terms, social media is used as the modern-day equivalent of word-of-mouth marketing but at an exceptionally quicker speed.