The term Work Type is used to describe work performed within the organization. In PSA, the combination of a work type + specific project = unique task.
Work Types are kept generic, to be performed by many departments within the company and may be applied to many projects. A task, on the other hand, is unique.
For example, customer support is a work type; but once customer support is associated with the project for ABC Inc., then it is no longer a generic work type, but a unique task for ABC Inc. The Work Type Customer Support may also be associated with several other projects.
Note: A work type may be associated to the same Project multiple times. However, if the Enable Work Type Uniqueness option is selected under the Defaults page (Setup > Work > Clients), then users won't be able to associate the same work type to the same project more than once, maintaining the task's unique identifiers as the project/work type combination.
For further details refer to: