By user data I mean anything an application needs on a user. For instance, the profile.json for every user account is public, like so:
https://gaia.blockstack.org/hub/1onTykegw5oYjsThkLfCZP6oqGNkkyebz/profile.json
Inside the profile.json, you can see different app “data buckets” within the apps[] array in the decodedToken field. Not all app “bucket” urls are stored in that array, but rather only the ones that require multiplayer (read: shared) data storage and thus have to be public.
Each of these data buckets store data that the app decides to put there. For instance, Graphite will store documents that the user creates and saves within its app “data bucket.”
More private information like your email would either be stored encrypted in the profile.json (at least, that’s what I would suggest) or simply stored in the browser state and have to be re-inputted every time you login (like what currently happens).
You can control where the data is hosted by creating your own Gaia Hub for your own storage back end, at least, in theory. There has been quite a bit of discussion on increasing the number, but currently most, if not all users simply use the one provided by Blockstack PBC because that’s the only one the official browser supports. People are moving to solve this though, thankfully.
Everytime you switch Hubs/Storage providers though, you have to update your zone file (which can be viewed through explorer.blockstack.org), which also costs money, and by money I mean whatever the BTC transfer fee is at the time (currently $0.62 USD). That and you have to (somehow) export all of your data out, and re-put it back into the new storage provider… it’s not easy, but maybe one day it will be.
(disclaimer: if you have no “name” and you transfer to a different gaiahub, the normal browser won’t know how to resolve your storage provider because it will default to the default gaia hub, and won’t get redirected. Maybe one day redirects could be implemented too, though…).