Adware or advertising-supported software is any software application in which advertisements are displayed while the program is running. These applications include additional code that displays the ads in pop-up windows or through a bar that appears on a computer screen. Adware helps recover programming development costs, and helps to hold down the price of the application for the user (even making it free of charge)—and, of course, it can give programmers a profit, which helps to motivate them to write, maintain, and upgrade valuable software.

Some adware is also shareware, as such it may be used as term of distinction used to differentiate between types of shareware software. What differentiates adware from other shareware is that it is primarily advertising supported. Users may also be given the option to pay for a "registered" or "licensed" copy, which typically does away with the advertisements. Other types of shareware include demoware, nagware, crippleware, freeware, and even spyware.

Some adware programs have been criticized for occasionally including code that tracks a user's personal information and passes it on to third parties, without the user's authorization or knowledge. This practice has been dubbed spyware and has prompted an outcry from computer security and privacy advocates, including the Electronic Privacy Information Center [1]. Other adware programs do not track a user's personal information. Often, spyware applications send the user's browsing habits to an adserving company, which then targets adverts at the user based on their interests. Kazaa and eXeem are popular programs which incorporate software of this type.

A number of software applications are available to help computer users search for and modify adware programs to block the presentation of advertisements and to remove spyware modules. To avoid a backlash, as with the advertising industry in general, creators of adware must balance their attempts to generate revenue with users' desire to be left alone.

See also: spyware, malware