At-will employment allows you and your boss to terminate your employment at your current workplace without notice and for no reason -- within reason. However, this policy is often misunderstood, and one state, Montana, disallows at-will employment, creating confusion for people from other states applying there for work.
Working in an at-will environment can change if you have a contract. It also does not allow bosses to fire you for illegal reasons, which means that if you suspect that your sudden firing was actually retaliation, or that it was done because of a personal characteristic such as gender or race, you may have a legal complaint and a court case on your hands.
A Written Contract
The United States, with the exception of Montana, allows at-will employment unless the employer and employee sign a contract that lays out termination and quitting terms, or if the employer has a policy that states what firing and layoffs require. For example, some universities require extensive documentation and attempts to remedy situations before an employee can be fired if that employee has worked for the university for a specific minimum amount of time. If you work for an employer who has you sign a contract that states they need to give you 30 days' notice before firing you for no reason and that you need to give them 30 days' notice before quitting, then those are the terms that you and your employer must fulfill. If you live in an at-will state, and you have no contract, you have to assume your employment is at-will.
In Some States, a Verbal Contract
In many cases, a verbal contract is binding and allows you to circumvent at-will laws. But the discussed terms must be detailed, and employees and employers both have to keep in mind that it is hard to prove or disprove verbal contracts when they boil down to they-said/they-said. In employment, employers have to be very careful not to promise that employment won't be at-will if they don't really mean it, and if they don't have a specific process for termination/quitting in mind. Applicants and employees need to record what they were told in detail.
At-Will vs. Unlawful Termination
In situations where a job is not at-will, such as in Montana or in employment situations where the employer has decided not to be at-will, that does not prevent the employer from firing an employee on the spot. However, the reason has to be specific and legal. An employer can't fire someone because they felt like it, but the employer can immediately fire someone who unleashed a barrage of racist language against another employee, nor can the employer fire someone because the employer doesn't want that person practicing a certain religion. The specifics of the firing process will vary; for example, the university in a previous example may need documentation and a disciplinary hearing.
However, if the employee is fired for insubordination, for example, but the employee thinks it was really retaliation for a complaint or really firing due to discriminatory reasons, then that employee needs to speak with an employment attorney regardless of at-will status. And anyone who thinks they have proof their employment was not at-will but who was let go for unspecified reasons needs to speak to an attorney, too.
If you have additional questions, contact a local employment attorney.