One of the advantages of using the online services of a company with lots of products is that one product can "borrow" features from another. This is especially true for Google. For example, Google has a Contacts app that lives at http://google.com/contacts but whose functionality is accessible via Gmail, Google Calendar and other Google services.
When we set up a meeting in Google Calendar, and invite people, we know where the data is coming from: Google's other app called Google Contacts. That's how Google conceptualizes Contacts, and that's how users do, too.
Google Buzz is the first Google product I can think of that appears to be a feature set shared by an invisible app. Where is the Buzz home page? Google Buzz seemed to be everywhere and nowhere, in Gmail, Profiles, Maps and Reader. Where does Buzz live?
Finally, we have an answer: Google Buzz is a feature of Google Profiles. When you use Buzz on Gmail, Maps and Reader, you're using a shared feature of Google Profiles.
This is how Google conceptualizes Buzz, and -- finally -- it's how we can think of it, too.
How do we know this? Well, it turns out that you can disable Google Buzz altogether, but if you do, your account on Google Profiles is permanently deleted.
All the services that use Buzz existed before Buzz came along -- Profiles, Gmail, Maps and Reader. But now that Buzz is here, only Profiles is deleted when you stop using Buzz.
In other words, you could use Profiles before Buzz existed without Buzz (obviously). But now that Buzz exists, once you activate Buzz, you cannot stop using Buzz without killing your Profiles account, too.
Buzz is a feature of Profiles. Fine. Why didn't they just tell us that from the beginning?