This is the US census I believe:
http://en.wikipedia.org/wiki/American_Community_Survey

If you don't send it back, they will come to your home. And they will interview your neighbors. On our street we were the only responders, and when the live humans showed up it was quite impressive. They had name tags, and signs on their cars.

The information from the census is very important, to get Federal aid for cities and states for elderly, disabled, and children.

A funny thing happened when the live interviews began. They had already been here once. Hubby returned home from work (he had a business audit) and one of these census workers approached him with her badge held out, so he whipped out HIS Federal ID and she jumped back! Then they both laughed.
She explained alot to him then, in fact. Since we have many empty homes in the whole neighborhood, the workers were interviewing everyone about the people who left, so they could figure out if they filled out a form from the location they moved to, etc. It is important not to overcount, or undercount, as you can imagine.