To start with, any applicant must submit proof of citizenship, most commonly a birth certificate. Followed by one's name, date of birth, social security number, place of birth, address, e-mail address and phone number. You're asked to provide any other names you have had, whether by marriage or a legal name change.
You are asked to provide information on your parents, including their names, birth date and place of birth. Other information includes height, hair color, eye color and occupation. Emergency contact information is required as well as the name and date of birth of your spouse. You're asked if you've ever been issued a passport and, if so, what happened to it. And, you're asked about your current travel plans.
Is that all? Not according to this report from, of all places Computer World: "In addition to passport applications and the related material, the system stores investigative reports that might have been compiled in connection with granting or denying a passport, or in connection with any violation of passport criminal statutes. It also stores court documents and administrative determinations related to passports and citizenship, as well as copies of birth and baptismal certificates, medical, personal and financial reports and details on arrest warrants of the person applying for, extending or renewing a passport and a person's Social Security number."