A parasiteware is a special form of adware which usually overwrites affiliate tracking links. Thus, affiliate commissions are paid to the software editor.
The parasiteware may be a software which allows users to collect cash back, to contribute to causes or to use coupons while shopping online.
Most often parasitewares are toolbars voluntarily loaded by users, but sometimes they may also be browser plugin or a type of "trojan horse" program.
Their behavior is viewed by traditionnal affiliate as parasitic because, often, they earn commissions without really influencing purchases by overwritting legitimate affiliates’ links.
More details on parasitic affiliate.
Some merchant affiliate programs don’t accept parasitewares as affiliates.