Not much, except for name and price. When a patent on a particular drug expires, other companies may apply to produce a generic version of the brand name drug.
A generic drug is a copy that is the same as a brand-name drug in dosage, safety, strength, how it is taken, quality, performance and intended use.
Since generics use the same active ingredients and are shown to work the same way in the body, they have the same risks and benefits as their brand-name counterparts. In fact, most state laws require that the generic drug be a generic equivalent to the brand name version.