this article has a great multi-part guide to g+.
It describes most of the functions of g+. as well as how to manage g+.
For example, there is mention of 'reading' circles and 'sharing' circles.
You would use sharing circles for publishing info and/or pics to friends.
A person could appear in more than one of your sharing circles.
On the other hand, reading circles are place where you want to consume info from others and you would only have a person in one such circle.
This is a neat way of managing the amount of info that you are faced with.
Please note that g+ offers 'circles', period. It makes no distinction between reading, sharing, caring or any other kind of circles.
The distinction and naming is strictly yours.
Read the article for much more info on how to get the best of g+.