The first noteworthy innovation of Google+ is the concept of circles. In real life, our relationships with the people we meet and interact with take on any number of shades in an infinite spectrum: some are hot, some are cold, some are casual, some are deeply personal. On Facebook, all of these people are simply known as "friends," despite the fact that many of them are something less or more.
The core of Google+ is letting you define your relationships more accurately by placing your contacts into groups. The service starts you out with circles for "Friends," "Family", and "Acquaintences," but you can make your social network as monolithic or stratified as you want—circles of 1000 and circles of one work just the same.
Once you've got your contacts into circles, it's easy to hone in on the people you're interested in and find out what they're up to. Google+'s main feed has a sidebar that lets you instantly switch between which circle's updates are displayed.
And it works both ways: you only have to share your posts with the groups you want to see them. Keep Mom, Dad and boss away from your happy hour updates.