along with the 23 million, he also said "Romney noted that if out-of-work Americans who no longer look for jobs were counted, the unemployment rate would be 10.7 percent," a comment that is pure urban myth.
Unemployment is actually figured from a household survey, as noted in this NPR.org blog and from household survey data.
The household survey data comes from Census Bureau workers going to people's homes and asking them if they are working, if they are looking for work, etc. The establishment survey comes from the BLS contacting businesses and government agencies and asking them about the total number of employees they have, how many hours they've worked, etc. The BLS can't talk to every single person or every single business so they talk to about 140,000 businesses and government agencies and 60,000 households, and they use that data to calculate the job situation for the entire economy.
The "
Employment Situation Summary" is a .gov site that explains the whole thing.