Lies, Damn Lies, and Statistics

Every quarter or so, Uncle will report so many jobs "created".  I wonder where those numbers come from?  ADP?  Businesses mail a form into Uncle?  Withholding tax? Presumable they are reporting on hires.  Every quarter we do some hiring and some firing.  Are the reported numbers the number of hires less the number of fires?  Which it should be.  Or do they just report new hires 'cause you get a bigger number that way? 
Inquiring minds want to know.