You get 15GB of free storage for Gmail, Google Drive and Google+ Photos and you can buy additional storage (100GB, 200GB, 400GB, 1TB, 2TB, 4TB, 8TB, 16TB). Until now, the storage was only shared between Google Drive and Google+ Photos and you got 15GB of bonus storage for Gmail. Now the entire storage is shared between Gmail, Google Drive and Google+ Photos.
Here's what happens after upgrading to the 100GB plan:
You'll get this message: "Thanks for purchasing a Google Drive storage plan. (...) Starting today, you'll have this storage shared across Google Drive, Gmail and Google+ Photos. To manage this plan, visit google.com/settings/storage. Remember that with the purchase of this plan, you have priority access to phone, email, and chat support from one of our specialists. Contact us. Thanks for buying Google Drive storage, and welcome to the cloud!"
If you have an old plan, you'll see some extra storage called "early adopter bonus". As Google says, "any storage purchases, promotions, or adjustments that have been applied to your account in the past will be reflected in your amount of unified storage. In other words, your storage space will not be decreased".