Those who are able and willing and legal to work but can not get a job are considered unemployed. The Bureau of Labor Statistics releases this and other employment data monthly.
There are basically three main types of unemployment that is considered natural:
Frictional: Displacement from a job when one is moving to another job.
Structural: Unemployment due to mismatching of location or skill type. Also, when a company is restructuring it may decide to cut cost by reducing staff size causing structural unemployment.
Cyclical: Unemployment caused by the business cycle. At lows in the economy, aggregate demand for labor is decreased causing firms to shed jobs and hire less.
Unemployment rate beyond a natural or accepted level indicates a slowdown or even decline in the economy. This non-accelerating inflation rate of unemployment (NAIRU) is one, where price-setting and wage-setting behavior is stable. It is arguable and has ranged from 4% to 7% historically, although currently, economists in the United States tend to agree on the range of 5% to 6%.