Any type of site can offer media as part of its content so what is a media site? I think media sites fall into two categories:
Sites such as Flickr.com, YouTube.com, and Blip.tv provide hosting services. They allow you to upload your media and share it with others. Other sites, like cbs.com, are focused on broadcasting their own media. Then there are sites that utilize the hosting services of Flickr.com but use their own site to display the images and put them into context.