Adware or advertising-supported software is any software package which automatically plays, displays, or downloads advertising material to a computer after the software is installed on it or while the application is being used.

Adware is software integrated into or bundled with a program. It is usually seen by the programmer as a way to recover programming development costs, and in some cases it may allow the program to be provided to the user free of charge or at a reduced price. The advertising income may allow or motivate the programmer to continue to write, maintain and upgrade the software product.

Some adware is also shareware, and so the word may be used as term of distinction 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 to do away with the advertisements.

There are concerns about adware because it often takes the form of spyware, in which information about the user's activity is tracked, reported, and often re-sold, often without the knowledge or consent of the user. Of even greater concern is malware, which may interfere with the function of other software applications, in order to force users to visit a particular web site.

It is not uncommon for people to confuse "adware" with "spyware" and "malware", especially since these concepts overlap. For example, if one user installs "adware" on a computer, and consents to a tracking feature, the "adware" becomes "spyware" when another user visits that computer, and interacts with and is tracked by the "adware" without their consent.

Spyware has prompted an outcry from computer security and privacy advocates, including the Electronic Privacy Information Center. 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 file-sharing programs that deliver target ads to their users.

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.

The Eudora e-mail client is a popular example of an adware "mode" in a program. After a trial period during which all program features are available, the user is offered a choice: a free (but feature-limited) mode, an ad-supported mode with all the features enabled, or a paid mode that enables all features and turns off the ads. If the user choose the ad-supported mode, Eudora becomes adware, although according to Qualcomm the program does not collect any information about user activity.

Programs with know adware included with them:

123 Messenger · 180 Solutions · 180SearchAssistant · Zango-Bonzi Buddy · ClipGenie · Comet Cursor · Cydoor · RealPlayer · Direct Revenue · Aurora (adware) · Ebates MoneyMaker · Gator-PornDigger! · WinFixer · Hotbar · ErrorSafe · Smiley Central