Seasonal unemployment is not a significant cause. Frictional unemployment deals with the freedom of workers to leave their jobs and take time to find a better opportunity elsewhere. Structural unemployment deals with the freedom of employers to reorganize their businesses, including the elimination of jobs. This could happen due to decreased demand for a product or to changing technology. This results in unemployed workers who have skills for which there is no available job.
Cyclical unemployment occurs when the economy goes into recession.
Workers are laid off and unemployment rises. After reaching the trough, recovery begins and workers are hired back. Only frictional and structural unemployment exist. Cyclical unemployment reaches its maximum at the trough. Full employment exists when only frictional and structural unemployment exist.