What I remember from the 2000 Census is that there is one of two forms you can get in the mail: a short form and a longer form. The short form is pretty bare-bones; the longer form has more detailed information to track demographic trends (has such information as race, religion, salary range, etc. all of which I believe were optional fields.)

Disclaimer: This was 10 years ago and my recollection could be faulty. It's always wise to take precautions, though.