Some people are surprised to find out that they are "at-will" employees. They may not know what the term means, but they have a feeling that the term is not beneficial to them. So: what does it mean to be an at-will employee?
First, it's important to note that many people are considered at-will employees. This isn't a special status that is reserved for a select few employees. At-will employment means that your employer can terminate the employee-employer relationship at any moment for practically any reason. They don't need to warn you, and they don't even need a particularly good reason.
However, they do have to respect your rights as an employee and they also have to uphold the terms of any contract that you have signed. These are just a couple of exemptions to the "free firing" that seems to permeate the idea of at-will employment.
Another example is public policy. If as a matter of public policy -- one that has been explicit or well-established -- you shouldn't have been fired under at-will employment terms, then your employer may have committed an act of wrongful termination. Your employer may also imply that a contract is coming soon and they you are in line for new terms of employment -- only to let you go. This violates the implied contract exemption and can be grounds for wrongful termination.
If you feel your employer acted in bad faith and violated your rights, consult with an attorney immediately.
Source: FindLaw, "At-Will Employment and Wrongful Termination," Accessed Mar. 2, 2017