|December 2013 Unemployment Rates, US Bureau of Labor Statistics|
U1 is the percentage of people unemployed for 15 weeks or longer
U2 is the percentage of people who have lost a job or completed a temporary job
U3 is the official unemployment rate and is the percentage of people who are unemployed and have actively looked for work within the past 4 weeks
U4 is the U3 figure combined with discouraged workers who have given up looking for a job until the job market improves
U5 is U4 plus marginally attached workers who are able to work and would like a job but have not looked for a job recently
U6 is U5 plus part-time workers who cannot find a full-time job
So, while the official unemployment rate is down to 6.7% in December 2013, this figure only includes people who have looked for work in the past 4 weeks and who are not employed part-time.
At 6.7%, that means that 1 in 15 Americans cannot find a job. Using U6, 1 in 8 Americans in the labor force cannot find a full-time job. 1.4% of the labor force have given up looking for a job right now, and 5% of workers are stuck in a part-time job unable to find full-time work.
UPDATE: My friend Michael points out that these rates are for the civilian labor force and thus do not include American military workers and many/most Federal workers.