They are in fact pretty similar in layout, but im noticeing its somewhat of a “mix” between facebook and twitter.
Basicly, you can add anyone on Google+ without a “confirmation” much like following someone on twitter, you can follow anyone (providing they dont have a follow request setup) So I can add you to my Google+ Friends circle if I want even if you don’t add me, so anything you post publicly that you dont care who see’s I can see, again a lot like twitter.
But unless you add me to your circles I cannot see anything private or anything that is not meant for my circle.
So say your in my friends circle, my family is in my family circle… I want to post something that i want my family to see like this cute picture of my daughter, but I don’t want people in my friends circle to see it, unless I add my friends circle in that post, only my family will see it.