The aftermarket, also called secondary market, refers to the market for products and services related to the original products/services customers purchased. Examples of aftermarket parts include spare tires for automobiles, blades for razors, extended warranty plans for electronic equipments, etc.
In many cases the aftermarket generates as much or even more profit for companies than the original product. During product development, it is very important to create a product line that will produce not just one central product, but accessories as well.
Industries that are well known for capitalizing on the aftermarket are the automobile industry and the computer industry. Quite obviously, for both industries, it is not just the car nor the computer that generates much profit, but the accessories that people buy to get more features.
For cars this can include everything from plush interiors to decals and even basics like sun shields. For computers, this can mean bigger hard drives to software upgrades and games.
Add-on services can also be considered part of the aftermarket, since these services are purchased only because they purchased the main product.
For example, in the auto industry, having a car automatically means getting car insurance. It can also mean getting service warranties, which usually come free for a certain number of years as part of the package.