This article will explain the difference between Groups, Jobs, and Departments. To see Jobs and Departments in the software you must have the Job/Department Costing module enabled.
The basic difference between Groups and Jobs/Depts is that groups are meant to be static - they should not change on a daily, weekly, or likely even monthly basis. They should be something like an employee's position, that will likely not change regularly. The group the employee belongs to is a required field when adding employees.
Jobs and Departments
Unlike Groups, with Jobs and Departments you can set an employee to have different Jobs/Departments each day on their schedule, or they can clock into different Jobs/Depts on the timeclock.
You can also edit their timecard to be for a specific Job/Dept:
This will show on their timecard, so if they work multiple jobs/depts in a day you can see the hours split between the different jobs/depts:
Difference between Jobs and Departments:
The only difference between a job and a department is that you can assign a timeclock device to a Department (but NOT a Job). This way if any employee clocks in on this device, it will clock them into the assigned deparment.