A “protected” applicant is an individual with one or more of the characteristics defined by Title VII of the Civil Rights Act of 1964 (race, color, sex, national origin, religion), is age 40 or older or has a disability.
If any of your application procedures tends to screen out certain classes of applicants (such as women or the disabled) in a way that has nothing to do with the essential functions of the job, you could be sued for discrimination.
Also, some states and municipalities have laws against discrimination based on sexual preference. Check the laws in your area. If such statutes are on the books, then gay and lesbian applicants, to the extent you can identify them, are protected candidates as well.
That’s the case even if you didn’t intend to disqualify such applicants. And that’s where it can be tricky for employers. Consider the Immigration Reform and Control Act and the ADA. An employer may be trying to follow those laws to the letter and yet unknowingly violate key provisions.