[]The guid tag uniquely identifies an item. For example, if a resource is published at a particular URL on one day but is archived at a different URL, the guid provides an aggregator with a way to link into a resource at its constant URL. Similarly, if a resource is edited, RSS aggregators that use the guid tag are smart enough not to redisplay the whole item to a reader.[/]
That's what I could find. guid is an id, where link is just the url