Legally what information is a former employer allowed to tell a potential employer? For example if you are fired from your job, can they tell the person that you were fired AND why?

Yes and yes. Despite a widespread and persistant belief that employers are limited to providing dates of employment, job title and salary, the truth is that with the exception of any medical information they may have, an employer may provide any and all information they choose to as long as it is true, that they have a good faith belief that is true, or that represents their honest and supportable opinion. They absolutely may tell a prospective employer that you were fired and why as long as they tell the truth as they understand it to be.

Is this true if you are in a union as well?

Yes, the law (or lack thereof) is what it is. I work in a totally unionized environment and I have yet to see a union contract that dictates to the employer what they may or may not say regarding a reason for termination of a member. But even if it did, and the employer violated it, it would be a breach of the union contract, not a violation of law.