Structural:
A given set of skills is not needed by an economy, so people with those skills cannot find employment
Frictional:
When people are in a set period of unemployment whilst they wait to get a job that is already secured, or when people are looking for their first job
Seasonal:
When there is unemployment as the jobs only exist at certain times of the year (e.g. retail jobs at Christmas)
Cyclical:
When the growth of the economy has slowed and there is no demand for the production of the workers at all