Up until now, Google has given you separate storage for Google Drive, Google+ and Gmail. Now, they've decided to do the logical thing and unify it to a single 15GB chunk.
One of Google’s great advantages over other services (DropBox being an example in regard to cloud storage) is that you already have it. In all likelihood, you have a Gmail account and if you do, you also have Google Play, Google+, Google Drive and all manner of other services. Everything is neatly gathered in one place, which is why Google realized it doesn’t make a lot of sense to compartmentalize that place. Previously, Google+ and Gmail have allotted you 10GB of free space, while Google Drive had 5GB. Now though, all of that data is being combined into an overall shared 15GB of free storage across all services.
Your data is all centralized and a handy pie chart shows you what's where
This is a massive benefit, simply because people use Google’s services differently. Someone who only uses Gmail a little, but Drive a lot, can now make use of all the space that was just sitting there gathering dust before. In order to help you visualize your space, Google Drive’s storage page has been updated with a handy pie chart where you can also purchase upgrades, starting at $4.99/month. The changes to Drive will roll out over the next few weeks, and Google Apps users are getting it too.