Most of us are employed "at will" in Pennsylvania (and, as I understand it, in every state in America except California).
There are basically only 2 exceptions to this rule: 1) you are part of a Union (in which case you can only be fired in accordance with the terms of the Collective Bargaining Agreement, which usually provide for "progressive discipline"), or 2) if you have a contract providing that you will be employed for a period of years unless you engage in "for cause" conduct that will result in your termination [(the most common example of this is found in professinal sports - if you have ever heard about a team still paying a player or manager who it terminated last year, that is because the player/manager had a contract for a period of years and was terminated for "poor performance" as opposed to because he/she engaged in "for cause" conduct].
Employment At Will - Defined
If you are employed at will, you can be fired for any reason: a good reason (you did not show up for work for 3 days, you embezzled money, you were late 15 times in a month, etc.), a bad reason (someone said something about you that was untrue and the company believed it, the boss is a jerk and does not like you, the boss likes someone more than you, you are incorrectly perceived as incompetent, etc.) or no reason at all (economic layoff, shut down of a department).
People generally understand that they can be fired for a "good reason", or no reason at all - it is getting fired for a "bad reason" that perplexes and angers us (and for good reason!).
The Exceptions to the Employment At Will Doctrine
There are exceptions to the "at will" principle in Pennsylvania (and throughout the United States) - the doctrine of wrongful termination (the attached Article gives a nice overview of the wrongful termination principle in Pennsylvania - if you fit into any of these categories, you should consult with counsel), unlawful discrimination or retaliation, violation of the FMLA or FLSA, etc. It is those exceptions that lead to litigation - it is those exceptions that we litigate every day in our office.
Otherwise, the battle about whether your termination was justified is waged in the unemployment compensation department, where the issue is whether the company can prove that you did, in fact, engage in the willful misconduct that was the bad reason for which you were (allegedly) terminated. Our experience in unemployment hearings is that an employer very often cannot actually prove you did anything wrong.
The at will doctrine is among the most difficult to understand because we all believe (and justifiably) that we are entitled to be treated fairly if we are dedicated employees. However, sadly, that is not the truth for most of us at will employees in Pennsylvania (or throughout the Country).