A silver account allows you to do download videos/demos, arcade games etc and the account is free but you can't play games online. The gold account requires a yearly subscription (there a few games out which have a free trial code in them). With gold you can do everything you can with silver AND play online.
There was a brief mention not long ago about gold users getting some demos/videos a few weeks before silver users could access them but I'm not sure if this is going to happen or not.