In the United States, the U3 Unemployment Rate is a measure of unemployment. It is defined as people without jobs, who have actively searched for employment in the past four weeks. This is the "official" unemployment rate, and the one usually quoted in press releases and official statements. For general purposes, this is the unemployment rate.
As the number "3" would suggest, there are several different unemployment rates, starting at "U1" and going to "U6". "U1" is a smaller number, excluding people who have had any type of work in the past 15 weeks. Each category includes people with a wider definition of "Unemployed", until "U6" includes people who are underemployed or discouraged. Although there are different ways to think about unemployment, the U3 rate is considered to be the standard measurement.