Google is eliminating its separate quotas for Gmail and other services, combining Gmail's previous 10GB quota with the 5GB allocated to Google Drive and Picasa/Google+ Pictures. That change will also affect Apps for Business users, who now have 30GB to use however they like.
Google will update its storage tracking page to include a pie chart demonstrating where your storage is used. Bear in mind that documents created natively in Drive don't count towards the total; nor do pictures smaller than 800 by 800 (or 2048 by 2048 if you upload pictures via Google+, as unlikely as that is).
Google began down this path last September, when it combined storage totals for Drive and Picasa (and trimmed the total amount of storage on offer at the same time). The latest change does have advantages for Gmail users, since it means you can now expand your available storage beyond the previous upper limit of 25GB, presuming you're happy to pay.
As with most Google changes, this will roll out gradually over a few weeks (it hasn't shown up on my account yet).