While Ron gave the basics, you should really check with a corporate law attorney to find out exactly how much your state law protects minority owners. Right to see financials, proportional share of dividends, etc. In California, if less than 20% (not positive on the exact figure), the majority partner can 'freeze out' the minor partner from the books, take all profits as salary, etc. Above that cut-off, you can use the court system.
Also, you need an equitable buy-out agreement signed in advance. Share value in a closely-held corporation can be manipulated by majority partner, i.e., made worthless.
Still trying to collect from a crooked bastard past partner,
Ken