Google Plus is currently only usable by selected users on a field trial basis. During this invite-only stage, Google+ has four components: +Circles, +Hangouts, +Mobile and +Sparks.
This is different from the +1 button that was introduced in March, 2011.
+Circles: share what matters, with the people who matter most
Not all relationships are created equal. So in life we share one thing with college buddies, another with parents, and almost nothing with our boss.
Google is trying to provide more privacy control.
+Hangouts- multi-user video chat system
+Mobile- Always-on geotagging and instant cloud uploading and caching to content sent from your mobile device