Unemployment is when people are unable to find jobs at the prevailing wages for their skills and experience. Economists refer to this as "involuntary unemployment."
After reaching a low of 4.1% in October, 2006, the U.S. unemployment rate has been trending upward, reaching 9.4% in May 2009. It will continue rising for several more months. Unemployment normally peaks a few months after the economy reached bottom and starts going up again.
Some people think that to get figures on unemployment the Government uses the number of persons filing claims for unemployment insurance (UI) benefits under State or Federal Government programs. But some people are still jobless when their benefits run out, and many more are not eligible at all or delay or never apply for benefits. So the Government conducts a monthly sample survey called the Current Population Survey (CPS) to measure the extent of unemployment in the country. The CPS has been conducted in the United States every month since 1940 when it began as a Work Projects Administration project. 
In some fields (such as technology) the unemployment rate for persons over 45 years old can be as high as 80%.
Three types of unemployment can be distinguished by their causes. Frictional unemployment is the normal churning of the job market; people enter the labor force from school or home, and spend time looking for the best job. Seasonal unemployment refers to jobs that operate at certain seasons, especially in the construction and tourism industries. Teachers in the summertime are not counted as unemployed. The "normal unemployment rate" (frictional plus seasonal) is about 3% to 4%. Unemployment insurance is designed to help people face frictional and seasonal unemployment. It permits them to keep their skills rather than take a low-skilled job.
Structural unemployment means a mis-match of jobs and workers. For example, there may be jobs in the cities and unemployment in mining or logging areas, but people are reluctant to move because of family ties. There may be jobs available in some fields but people who are unemployed lack the necessary skills. This is a structural problem solved by migration, job training and education. Some structural unemployment is caused by policies such as the minimum wage.
Finally--and most serious--is cyclic unemployment (as in 2008-2009 and the Great Depression), when the whole economy is sagging and businesses lay off tens of thousands of workers every day. A few new jobs are created every month but they are overwhelmed by the jobs that are lost. In February 2009, the U.S. economy lost 651,000 jobs net (that is, jobs that disappeared outnumbered jobs that were created by 641,000). Through April 30, 2009, about 5.7 million jobs were lost in 2008-9.
At election time, unemployment is often portrayed as a great evil, but Milton Friedman said,
- All of the progress that the US has made over the last couple of centuries has come from unemployment. It has come from figuring out how to produce more goods with fewer workers, thereby releasing labor to be more productive in other areas. It has never come about through permanent unemployment, but temporary unemployment, in the process of shifting people from one area to another.