User Generated Content is a digital marketing term describing the process where your customers generate marketing materials for you. This is done through blog posts, social media, or other forms of word of mouth. It's especially powerful since it's consumers talking to other consumers without the company in the way (which makes it more authentic and also cheap).
MMO's are by their nature user generated content as the game developers create a framework for people to play in and then the players breath life into that world by their actions. EVE Online is the probably the best example of this as illustrated by this new trailer for EVE Online which is all player generated content.
The trailer features real player voices and real situations that were submitted to CCP for the trailer that have been beefed up with some music and some fantastic visuals. It's a pretty cool experience but it does kind of gloss over some of the tedium and grinding that leads up to a lot of these moments.