Task types
Task types are primarily used to determine how a task is planned.
An adhoc task is usually only planned for a short period of time (like "install windows in the house"), whereas repeat - exact times and repeat - loose times are intended to be planned regularly for long periods of time (like "clean the windows every 4 weeks").
For details about planning, see the Planning tab.
Adhoc
This is for tasks that need not be done regularly. If a task need planning then you will often use the Planning module to plan the task to a specific employee on a specific date.
Repeat - exact times
This is for tasks that should done on exact dates by a specific employee, like every third Friday from 10 to 12, or every day in the first week of each month.
Repeat - loose times
This is for tasks that should be done regularly, but it is not important exactly when or by who. These tasks will often be closed in the period when they are not being done, and are then automatically reopened when it is time, with a deadline a few days ahead.
Project
A task of type project may have subtasks. Hour registration, planning etc is done on the subtasks.
This type is not available if the Project module is installed.
Be aware of that only ad-hoc can be a subtask to a project