Google+ or Google Plus is Google's Social Network to introduce services such as Circles, Hangouts, Sparks, etc..
For those of you who have already joined Google+, but for some reason you want to remove it and go back to Gmail without Google, here below is the way to remove it.
How to Delete Google Plus
• Login to your Google+
• Go to the Account menu located in the upper right (click on the profile picture, then click on Account)
(Click on image to enlarge)
• At the next display select: Delete Google+ content
• Then click the Remove selected services
May be useful.