Your instance knows

• Your IP address
• Your E-Mail (if provided
• Your password (salted of course)
• Your session type (browser, app, etc)
• Your interactions (posts, comments, saved, upvotes, downvotes, etc
• Your social graph (following and followers, etc)

Everyone in public knows

• Your session type (optional)
• Your interactions (posts (if set public), comments, saved (depending on service), upvotes, downvotes, boosts, etc)
• Your social graph (may be optional depending on service)