Post to the public = a Facebook public account
Post to one or more Circles = a Facebook private account with fine tuning.
Note that you don't have to use their pre-defined Circles. You can delete them and make up your own.
Note also that one of the pre-defined Circles is labelled "Following". This seems to amount to a suggestion that Google+ users emulate one of the characteristics of Twitter, and it is diametrically opposed to how a private Facebook account works, where all relationships have to be mutual. At the same time, a fair number of people are using Google+ to write extended posts, which can't be done on Facebook or Twitter. Some people are essentially using Google+ as a blogging platform.