Employees are people you want to assign items to or otherwise want to track.
Users are people who can sign into the Tracker. An employee can be a user but doesn't have to be. A user can be an employee but doesn't have to be.
Employees can be primary or secondary assignments (see Tasks and Secondary assignments in Preferences).
Employees can be imported.
ADD, EDIT AND DELETE EMPLOYEES
Every employee needs a 'number' (which can contain letters, spaces and punctuation marks). If you already have employee numbers, they'll work in the Tracker. If you're interested in integrating Tracker data with your accounting system, now or in the future, use the numbers in that system. But if you're creating new numbers ...
The Employees table contains a large number of user-definable fields. All fields, whether user-definable or not, can be given custom names. So you may see a field in your Employees screen that's not in this list: