Google+ (Google Plus) serves as social network enabling users to connect with friends, family, and professionals to share data, photos, send messages, and engage with content.
The Google+ Button allows users to "like" your content and makes it more relevant in Google searches, improving your SEO.
Google Circles helps categorize your follows and who you follow. You can name them however you choose and you can choose to share only with people in specific circles .
Google Extended Circles means that whatever you share appears in someone else's circle and can be shared by their followers.
Google Communities is a forum created by users, companies, and brands to discuss particular topics or services or to share information in general.
Google Hangout is a service on Google+ that allows users to video chat, message, or phone call with friends and associates.
Google Streams is a the news feed showing content that has either been posted publicly or by people within your circles.