Are employee and user the same thing?

Employee and user are not necessarily the same thing. An employee is a person who works for your company. A user is a person who has an account allowing them to access your xTuple database. There are scenarios where database users would not also be employees. For example, you may give user access to people who are not your employees (e.g., IT consultants). Conversely, there are also scenarios where employees wouldn't need user accounts. For example, you may have employees whose job doesn't require them to use a computer—and so there would be no need for them to have a user account.