The document discusses different types of unemployment:
1) Natural rate of unemployment refers to unemployment that exists even during economic booms due to frictional and structural factors.
2) Cyclical unemployment fluctuates with the business cycle and refers to unemployment during recessions.
3) Frictional unemployment results from the time it takes for workers to find suitable jobs as worker and job characteristics change.
4) Structural unemployment occurs when there are insufficient jobs in certain sectors for all who want to work, such as due to minimum wages, unions, or efficiency wages above market levels.