Unless there is an employment contract or union agreement all employees are presumed to be "at will". At will simply means the employer/employee relationship can be terminated by either the employer or employee at any time without notice. You can terminate an employee for cause or for no cause and the employee can quit.
There are exceptions which may vary slightly by state. If, for example, your employee manual includes specific steps to be taken before an employee is to be terminated it can create an implied contract. Employee manuals are important but do not make them so detailed they can be used against you.
Note that you do not need a good reason to terminate someone or even any reason at all. You can do it "at will" unless it is done in such a way that it discriminates against a specific protected group as defined by law. You cannot fire an employee because of race, religion, sex (including pregnancy), natural origin, age, disability or genetic information. ( http://www.eeoc.gov/employers/coverage_private.cfm
) These laws generally apply to employers with more than fifteen employees. While I suggest you do not ignore the law if you have less than the required number of employees it does make complaints more difficult.
The "at will" doctrine is one reason that you should not go into detail when firing an employee. If you say you are terminating an employee for cause then they can argue whether the actions justify the termination. Without a stated reason there is no argument
Original content copyright 2010 Thomas Robinson