As U.S. citizens we have certain rights, freedoms and responsibilities. The Census falls under the responsibility category. And yes, it's backed by law. If you want all the facts, they are easy to find:

security issues:
More Security Topics - 2010 Census

and

sample form:
The Questions on the Form - 2010 Census


and
questions answered:
Questions You May Have - 2010 Census


Honestly, I don't understand the consternation. This is no big deal. We are lucky to live in a country that gives us so much, yet, as compared to so many other countries, requires so little. Enough of the whining!