In this climate of litigation, the probability is that most organizations that have been given the proper legal advice will only disclose three pieces of information about you, no matter the quality of the job you did:
Some information is absolutely prohibited from disclosure:
Your driving record is public record and can be released without your consent.
Legal Realities
Your previous employer does not have to answer any questions. (Remember the First Amendment? Freedom of speech is also freedom not to speak.) The past employer bears significant liability for a defamation action by you if it reveals any information that is both (1) untrue and (2) damaging to you. If the answer prevents your getting the job, it is damaging to you.
As stated at the beginning of this answer, most organizations, particularly large organizations, governmental organizations, and smaller organizations that have been given good advice by lawyers have very strict policies governing what information is allowed to be released about you.
If you are unsure about a potential reference there are several companies that will check your references for you and give a detailed report. Put "reference check" in a search engine and you will discover information about many such businesses.
If the referee is very pleased with you, (s)he may be inclined to give a glowing reference, but, again, the strict policies apply, and in today's era of litigation, the probability is that the organization will remain silent except for the three pieces of information mentioned at the beginning of this answer. Further, the referee that gives out the "glowing" reference may be exposing her or his organization to liability should your prospective employer suffer damages because of you--the present employer may bring an action against the referring employer for failing to disclose the negative information.
One way people gather information about applicants while reducing liability is for an individual person working for the prospective employer, acting, supposedly, "as an individual," to contact the referee at home in her or his capacity "as an individual," so that the referee is not acting as an agent of the organization. That way, sometimes, information flows more freely, but an individual contacted will still fear saying much that is negative about you for fear of a legal action by you, as already described, or a governmental entity, such as the Equal Employment Opportunity Commission (EEOC) or your state's version of the EEOC.
This is a complex topic. The above is, admittedly, a simplification. We recommend you seek some additional information.
There is a good and readable summary of the legal issues (albeit for California only) here:
http://www.esrcheck.com/ESRPublications/Reference_Checking.html
Think of items that they can't legally ask you in an interview: ie. religion, age, sexual orientation, medical history, and general other personal questions whose answers could lead them to not hire on discramatory grounds. Pretty much everything else is fair game.
A potential employer is limited in the types of questions he can ask your current employer. Generally, he can only ask things like if he would ever rehire you and if you are currently working there.
Background checks are unregulated.
Is a previous employer allowed to tell a prospective employer you were fired when you were not in Nevada?
You dates of employment and what position you held.
Yes. The legal implications would only be if they lied and caused you harm.
If a prospective employer calls your previous employer, the previous employer can say whatever they feel is true. If you don't want a bad reference, you may not want to put your previous employer on the application.
A past employer may give a prospective employer an overview of the employee's employment record. They can give their opinion about the employee's character.
A prospective employer may be interested in your health because many employers pay a portion of their employees' health insurance. Health insurance premiums may be higher if you are in poor health or a regular smoker. However, a potential employer is not legally allowed to ask questions about health during an interview.
I don't know about in California but I heard that an employer is not allowed to tell anything about an ex employee unless used as a reference. If you just use the employer as a previous job all they are supposed to be allowed to verify is that you did work there and the dates. However if you put them down as a reference and a prospective employer calls them they can elaborate on your character as well.
Yes. Legally, they are allowed to say anything that is factual and accurate.
All employers can ask that with zero liability.
prevent discrimination
This shows that your employer allowed you to leave on good terms. It is like a recommendation letter and allows future employers to feel more confident in hiring you.