Microsoft introduced quite a few changes with the release of App-V 5. Two of the main tenets of App-V have been isolation and redirection. App-V 4 isolated as much as possible while App-V 5 relaxes several of these isolation barriers which makes interoperability a little easier.
Tim Mangan put together a quick 16 minute video that discusses this change as well as others. This video is easy to follow and is more of a short presentation rather than a walk through. The video is easy to follow and explains some of the past App-V designs/history as well as what's new.