It's worth noting that sometimes you only have content that differs for your "logged in" users. Or, content that is the same with the exception of actions that a user has taken on your site. Say, in the case of a shopping cart.

In that use case, you might find that one cache covers everything except their shopping cart preview. This is where ESI (Edge Side Includes) come in. I've never used it in practice, but the idea is similar to server side includes of yesteryear. Here's a good article:

http://blog.redfin.com/devblog/2010/05/esi_and_caching_trick...