What data does the chip actually contain? Is it an identifier linked to a database or does it actually contain your personal information? One reason I dislike

What data does the chip actually contain? Is it an identifier linked to a database or does it actually contain your personal information? One reason I dislike biometric passports implementing RFID is because typically most suffer from poor implementation, it's like a one stop shop for identify theft for any nefarious type within a few meters of you.