Public Data vs Private Data in Trello
We don't want you to get a surprise that you've shared something you intended to keep private. You have full control over the visibility of your data, and all defaults favor privacy.
You have the ability to make boards and teams private, in which case only added members will be able to view the board or team. Actions on private boards and teams will not show up in profiles. Private teams and boards are not visible in search.
Your member profile is public. That includes your full name, username and bio, not your email. (If you set your full name to be your email, people will see that.) Your Profile page indicates each item's visibility status, as either Always Public or Always Hidden. Your avatar will be public to the internet, but you always have the option to remove the photo in favor of initials only.
Other users will only see actions on your public boards (unless they are part of one of your private boards or teams). Boards pages will list only public boards and teams.
See also Security, Backup, & Data.