One of the key ideas behind Google+ is sharing selectively, and the feature that fuels this concept is called Circles. Basically a Circle is a group of people. When you first join Google+, there are a few established Circles, including Family, Friends and Acquaintances, but you can delete these if you choose, and create as many additional circles as you like. Do you have a group of Work Friends? A Bridge Club? College Alumni who like to keep in touch? You can create a separate circle for each and then, when you have a message or photo or link to share, direct it to as many or as few of these groups as you like. Alternatively, you can make your post public for anyone to see.
This contrasts significantly from the Facebook approach, where everything you post is visible to all your "friends" unless you go out of your way to block their access individually. You can also create Facebook groups, but then you have to post separately to each group you want to reach, and it requires administrative effort. The Google+ approach is a much better fit to the way most of us organize our social lives, casually sharing information selectively rather than shouting it for the world to hear.
Another benefit of the Circles approach is the ability to filter incoming posts based on who sent them. With a click you can see only posts from members of an individual Circle, a great way to cut through the clutter.