One of the things that the BLS tracks in the unemployment data is the reason that people are unemployed. You might think that the reason someone must be unemployed is that "they lost their job", right? Well, no not everyone. Some people do lose their jobs by being laid off or terminated, other people finish temporary jobs, some people re-enter the job market after being out of the market for a while and still other people are newly minted college or high school grads entering the employment market for the first time.
Here is how the mix looks for October 2011 :
Between new entrants, reentrants and people who left their jobs that equals 42% of the people who are unemployed. Interestingly a pretty large portion of the unemployed did not get fired or laid off.