Entering hours using Timesheet is like maintaining your time spent data against various tasks on day to day basis. These hours for a given task goes into a fields called "Time Spent" which is different from Actuals. This field is editable only from Timesheet module (i.e. can not be manually edited from task itself).
Using timesheet you can view and enter your week wise timesheet against applicable tasks, navigate backward to earlier weeks to view / update your timesheet data. You can also update Task Status and To Do from Timesheet itself.
Also, there are Timesheet reports which can be generated from Reports menu to view team members timesheet data for given date range and in given format as required.
Timesheet option is available inside the Home menu, I am not sure whether Workspace Admin need to enable or the process to enable this option.