User-generated content(UGC) is also named as Consumer generated media (CGM) or user-created content (UCC), which means the user created the content of various kinds of media.
UGC is developed with the birth of web 2.0 technology.UGC's concept is originated in the Internet field, that user can upload its own original content through the Internet platform to display or provide to other users. It encourage users not just download information form Internet but also upload. Everybody can make a content and the role of customer changes to both customer and producer.Customers like to read customer reviews because it gives them confidence in a product or service beyond a marketer’s hype.
Different Type of UGC website
Social Networking
Face book, My space.
2.vedio and picture website
YouTube, Flicker, Viemo.
3Wikis. Wikipedia
4. Forums and Blogging.
5. Video games
6.Maps and location systems

Advantages
Firstly Gather information and content fast. UGC can speed up the process of search engine rankings simply because while you’re not having to take the time to post new content, users are doing it for you. Secondly UGC attract users to visit the website, increase the content quantitatively.It also encourage user to be creative.
Disadvantages
The problem with user-generated content is the loss of control it involves. For example YouTube has had with repeated uploading of copyright-infringing videos. How do you get your users to generate the kind of content you want them to produce instead of the kind they want to produce?[1] Another example is the negative reviews from Amazon. Some user even did not read the book but give criticism comment "Amazon needs to step up and delete these non-reviews of books. At the very least, they should not count in the book's star ratings."[2]
ugc25feb2008.jpg

Reference
1.2. The Downside of User-Generated Content Web outfits that thrive on content from users have no control over what that content will be—often to their great consternation By Mathew Ingram
http://en.wikipedia.org/wiki/User-generated_content
http://www.ugcprinciples.com/
http://www.wisegeek.com/what-is-user-generated-content.htm