Only what the lessons need: your username, the nickname you chose, a scrambled (hashed) version of your password, and your learning progress (XP, badges, stars, which lessons you finished, language and theme).
No email address, phone number, real name, location, or tracking. No adverts. Nothing is shared with other companies.
You, and the site admin (who can see your nickname, username, XP and badge count, and can reset your password or remove your account).
You can change your name or password, or delete your account and all saved progress at any time from My account.
Students: please use a nickname and a password you don't use anywhere else.