Adware refers to software that is free to acquire and use, but also includes ads, sometimes ads which use network and dynamically change, and therefore can occasionally be vulnerable to malvertising and malware. While considered freeware, the fact that they require the user to view advertisements in order to use them makes them inherently less free. It differs from shareware in that the advertisements are part of the experience and the user may use the software for any length of time, whereas with shareware, the user must eventually pay for the software to continue use.
In a famous instance, the previously popular opnen source repository hosting site, Sourceforge, was caught wrapping open source software with adware against their will, leading to backlash from the community.
In an of itself, adware is merely a business model that some software vendors use to support their development. Often enough, vendors will offer multiple versions of their software, some ad-supported and others for purchase only. Adware is very common the mobile platforms Android and iOS. Famously, the Kindle tablets from Amazon have ads on the home screen by default that the user must pay to remove.
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