I have just left my job and am about to embark on a job search. I'm worried about what my prior employer will say if someone calls for a reference. How much information can my former employer give?
Your former employer is free to pass along job-related information to any prospective or future employer. Your employer can share facts and good-faith opinions with other employers, but cannot defame you -- that is, say things about you that it knows aren't true.
The former employer is on safe legal ground if he or she acts in good faith and does not act maliciously. Still, employers are often skittish about saying too much to prospective employers who call fishing for information for fear of being sued by former employees.
Your best strategy is to clearly let your former employer know that you are aware of your legal rights if he or she exceeds the bounds of good faith in making a reference -- and that you will take immediate steps to exercise those rights if necessary.
If you are truly worried about how those facts and opinions may spin out, try to bypass that particular employer and give the names of other people with whom you have worked in the past with happier results. That may include people in volunteer organizations or clubs.