At-will employment means that you or your employer may terminate your employment at any time without any explanation. You will find at-will employment policies in every state except Montana.
The opposite of at-will is contract employment. If you do not have a contract, then you are an at-will employee. California Legislative Information explains that you might still have a claim for wrongful termination even working at will.
The key to wrongful termination of at-will employees is there is some type of discrimination present. It could be direct discrimination, such as firing you for your religious beliefs, or indirect, such as firing you for standing up for someone else who was a subject of discrimination.
Wrongful termination always involves an illegal act by your employer if you work at will because any other reason for letting you go falls under the at-will umbrella and is something you cannot make a claim against.
Remember that the beauty of at-will employment is that you can quit at any time. If you do not like the job or you do not like how the employer treats you, you can quit without any explanation or notice without fear that the employer can retaliate. It is this same concept that protects your employer in most termination situations.
If you feel your firing occurred due to discrimination or without a legal basis, you can consider filing a wrongful termination claim. Keep in mind that you will have to prove your case, which can be tricky. You may decide to seek other claims, such as wages owed to you that your employer did not pay, which is much easier to prove.