CIRCLES : Google+ circles make sharing online very similar to the real life sharing. Share what y
ou want only with whom you want.
SPARKS : Facebook is family(social) graph while Google+ is for interest graph. Sparks are an other way of finding data about your interests. They are not awesome as of now. But once Google+ starts incorporating best google+ feeds into them, they will definitely be a feature to watch out for.
HUDDLE : Huddle seems to like a group chat with your circles on Android. I am trying it out now.
HANGOUTS : Hangouts are group video chats with your freinds. It is a great way to hangout with your friends.
SETTINGS : If you are a kind who feels Google+ is difficult to use watch this video.