The government defines those who want to work as people who have actively looked for work within the past four weeks and determines the number of people currently unemployed through a monthly survey called the Current Population Survey.
People can be unemployed for many reasons:
They quit their position and are looking for a new one.
They were laid off due to lack of work and haven't yet been rehired.
Their company reduced the work force, and they are seeking a new position. This can be due to a local condition, when the company closes a plant or division, or a national condition, when the economy slows and many companies reduce their work force.
They have recently returned to the work force - perhaps from pregnancy or attending school - and haven't yet located a position.
The need for their skill set has gone down, and there are limited positions available, which may lead to unemployment until they train for a new position.
Technology has reduced the need for their type of position.