Simon, thats Google's social network. As its developed by Google is deemed to add the most weight to your website in terms of SEO amongs the rest of the social platforms, if used correctly.
If you have a blog and post on there regularly, you can then publish the content on the G+. If your content is engaging and draws attention, ie comments and +1s this can prove beneficial to your website. There is much more to it though.
Some find it better to stick to the good old networking, repeat and referrals