Adware or advertising-supported software is any computer program or software package in which advertisements or other marketing material are included with or automatically loaded by the software and displayed or played back after installation or in which information about the computer or its users activities is uploaded automatically when the user has not requested it. These applications often present banner ads in pop-up windows or through a bar that appears on a computer screen.

The Opera web browser is a popular example. Adware helps some developers recover programming development costs, and it may allow the software to be provided to the user of the application free of charge or at a reduced price: due to the advertising, the programmer may still profit from the wide use of their work, motivating them to write, maintain, and upgrade the software product.

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, loyaltyware, 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 knowledge or consent. These programs are often called spyware; however, the word adware is sometimes also used to describe spyware.

Spyware has prompted an outcry from computer security and privacy advocates, including the Electronic Privacy Information Center [1]. 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.

Adware programs other than spyware do not invisibly collect and upload this activity record or personal information when the user of the computer has not expected or approved of the transfer, but some vendors of adware maintain that their application which does this is not also spyware, due to disclosure of program activities: for example, a product vendor may indicate that since somewhere in the product's Terms of Use, there is a clause that third-party software will be included that may collect and may report on computer use, that this Terms of Use disclosure means the product is just adware.

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.