Adware (advertising-supported software) is any software application that plays, displays, or downloads advertising content to a user’s computer.
Typical features are pop-up windows or banners, changes to home page and search engine settings in the web browser, and so forth. Some adware is installed with the computer-user’s permission: for example, during the installation of a legitimate application with which the adware is bundled. This is the case with various dubious toolbars.
The Anti-spyware Coallition defines adware as a type of advertising display software that delivers advertising content potentially in a manner or context that may be unexpected and unwanted by users.