Google Plus is Google's new social network. But why start a Google Plus account when you already have a Facebook account? Well, Facebook was developed on the premise that everyone is your "friend" which isn't how your social circles work in real life. Google plus is built so that you can intuitively break up all your connections into "circles" and treat each circle separately. (Facebook allows you to do some of the same things but the privacy settings are confusing and constantly changing.)
Google Plus has a lot of slick features, but the biggest reason to open a Google Plus account is that if you use a lot of Google products you will inevitably get one some day.