Aftermarket, both in R/C and full scale, refers to second- and third-party manufacturers who produce accessories and upgrades the original manufacturer did not. It is also one of the main reasons the R/C hobby is so strong and growing constantly as consumers seek to increase the performance and/or durability of their models. Not only do aftermarket companies aid the performance aspect of the hobby but they greatly help you to be able to call you model your own. Many of the primary manufacturers work closely with the aftermarket in developing parts. Examples are Traxxas and their relationship with Novak Electronics and Associated's relationship with MIP.
Like any good business, R/C aftermarket companies are sensitive to market trends and adjust their offerings to what is in the consumer's best interest. For example, a popular upgrade for the Traxxas Revo stadium truck is the addition of a 1/8-scale buggy wing. This has became such a desirable upgrade for many Revo owners that some aftermarket companies make such a wing kit specifically for it.
As you the modeler and consumer continue to seek ways of upgrading your models, so too will the aftermarket companies follow suit with new ideas and innovations to help further your enjoyment of the R/C hobby.