Sharing
When sharing content in Google+, you have some different options for who to share it with:
1) You can share it to individuals by autocompleting their names or entering an email address, and only these people will see your post.
2) You can share it with one or more circles. Only the people in the circles you chose will see the post.
3) You can share it to Extended Circles, which means your circles, and all of your circles' circles (kind of like friends-of-friends).
4) You can share it publicly, which means anyone can see your post.
5) Or, you can share to any combination of the above.
Consuming
With any of the sharing choices above, people will only see your post in their stream if they have you in a circle. So even if I share to my "best friends" circle, my friends won't see my post unless they also have me in one of their circles (or if they go to my profile).
Conversely, if I want to read my best friends' posts, just having them in my circle isn't good enough. They need to share their posts to a circle that includes me.
What does this mean for you as a Google+ user?
You know all those random people adding you to circles you? Don't worry about them, they'll only see your posts if you post publicly, or (maybe) if you post to your extended network.
Fine-tuning your circles is really the key to making the most out of Google Plus.
And, as most of you know, Google Plus is only about a week old! We're listening to all your feedback and writing code as fast as we can
This is one of the most energetic and passionate teams I've been on over my 6 years at Google. Expect a lot of great improvements, big and small, rolling out on a continual basis.