Think of Google Reader as a 3rd party sync API, and a variety of tools use this API. lets use Twitter as an example. Google Reader is Twitter with their API

Think of Google Reader as a 3rd party sync API, and a variety of tools use this API. lets use Twitter as an example. Google Reader is Twitter with their API Reeder (or your RSS reader with support for the API) is like Tweetbot. This isn't a great example, because read status of feed items and all that is handled by Google Reader. But it's close.