Google users get 10GB free storage on Gmail, and 5GB for Google Drive and Google+ photos. If you needed more storage, you had to sign up for a 25GB or 100GB or 200GB monthly plan, for $2.49 or $4.99 or $10.99. What Google is now doing, is allocating a 15GB unified storage to users, for them to use anyway they want.

Users can use all the storage in Gmail, or for Google Drive, or Google+ photos, or any combination of the three.  If you had upgraded your storage, there is no longer a 25GB cap on Gmail; any additional storage you purchased, applies to Gmail as well.

Google Apps users get  30GB combined storage  for Drive and Gmail now. Google Apps users too, can primarily use this storage towards Gmail, or for Drive.

[Image courtesy: Google]