When most people think about a contract, they think of a formal written document. And many contracts do look that way. There are other kinds of contracts, however. You and your employer can come to an oral agreement that is never put in writing, and it may still be a valid, enforceable employment contract. And, if your employer has ever promised you something, that may have created a contract as well.
If you fit into the following situations and are fired for questionable reasons, then you might consider talking to an attorney:
For more information on employment contracts, see Nolo's article Employment at Will: What Does It Mean?