You specify who gets to see each of your posts by selecting the “circles” the post is visible to. You add people to “circles” and when you post, you choose one or more circles who will see it.
And if it’s just a certain person who is already in a circle, but you don’t want to share it with the whole circle, just type in that person’s name.
To see what’s visible to a particular person, go to your profile and enter the name of the person in the “view profile as…” box.

@tom_g got it. You adjust the settings when you post. I was annoyed at this at first, thinking I had to do it every time rather than having a default like facebook does. However, after you do it once, the next time you go to post google+ remembers what you selected last time and does it automatically for you. You only have to change it if this particular post you want different people to see.