Google+ is a social networking site just like Twitter or Facebook that is operated by Google. It was launched for the first time on June 28th, 2011 for the testing phase though users were strictly advised to invite friend on their profiles who were over 18 years of age. See more details on http://en.wikipedia.org/wiki/Google%2B.
Google+ is the latest social networking site from the makers of google. They're trying to make it a new Facebook but so far it is private since you need to be invited to use the site or have a gmail account :)