It used to be that you'd have 10GB of storage for your Gmail account, and 5GB to share between your Google Drive and your Google+ pictures. Not anymore: Google just unified all three into a shared pool of 15GB.
This is great because now, you'll be able to make better use of all that space you weren't using before. Chances are, you weren't coming close to your 10 GB Gmail limit, or could clean out plenty of old emails if you are. And now, that space will be fair game for your Drive account.
The unification is going to come along with an update for the Google Drive storage page, to help you figure out how much you've got where, and if you decide to buy more storage, it's up for grabs across all three services. The changes will be rolling out over the next couple of weeks. [Google]