A generic medication is a medication which is manufactured by a company which was not the innovator. Once the patent on a medication ends, a generic company must demonstrate that its product has the same active ingredient(s) and works just as safely and effectively as the brand-name version before it becomes available on the market. The generic medication should provide comparable therapeutic effects as the brand name medication.
The only differences between generics and their brand-name counterparts is the generics are less expensive and may look slightly different (e.g. different shape or size or color), as trademark laws prevent a generic from looking exactly like the brand-name drug.
Generics are less expensive because generic manufacturers don't have to invest large sums of money to invent a drug. When the brand-name patent expires, generic companies are allowed to produce and sell generic versions at a lower, more competitive price.