TimeSheet List report can be termed as a mirror image of employee's TimeSheet which can be use for printing purpose. You do not  have the ability to modify the output of the report data.
 
TimeSheet Detail Report shows detailed description of time entered by employees against a Project, Tasks, and Sub Tasks.  This report can be used for Billing purposes.  You can modify the output of the report data.
 
Note: The timesheet list report cannot be run based on clients.