Any text, images, video or audio which is added to a web site by one of the web sites visitors is considered to be User Generated Content (UGC). UGC is extremely important on many web sites nowadays as it provides a number of benefits.

1. it provides more content at little or no expense to the web site owner
2. it can add a degreee of credibility to the opinions on a web site e.g. book or restaurant reviews
3. it increases interactivity between the web site and its readership

On the down side, it can require a lot of monitoring in order to ensure that the web site is not abused.

YouTube is an example of a User Generated Content web site.  YouTube simply provide a framework for all of the videos. They do not actually create any video themselves. All of the content of the YouTube website is created and uploaded by its users.

Other examples of User Generated Content include the comments on a blog or newspaper web site, questions and discussions on forums and bulletin boards, items for sale on eBay.