The unemployment rate most cited in the media is called U3. It is is calculated by dividing the number of Americans who are not working but who looked for work in the past four weeks (the unemployed) by the number of working-age Americans who are either employed or unemployed (the labor force).
Note, that if you're out of work but haven't been looking for a job in the past four weeks, you're not counted as unemployed, or even part of the labor force.
A better way to gauge the job market is the labor force as a percentage of the population, called the labor force participation rate. And recently, it has plunged, partly because many have given up looking for work.
But three big factors for the falling participation rate have nothing to do with the recession: One, America is aging. Two, men have been leaving the labor force consistently for 60 years. And three, people aren't working because they're in school.