An adware is a software delivering ads and which is mostly installed on desktops or other devices without the user being aware of the process. Ads may be displayed either embedded in the application or in a browser window.
Revenues related to the ad impressions or clicks are paid by advertisers to the software editor/developer.
An Adware can be legitimate or malicious.
Legitimate adwares are offered as sponsored freewares or as free versions of traditionally paid softwares. In this case, adwares are installed and they operate in transparent ways.
Non legitimate adwares are often installed without the user knowing it (the user unwillingly install the software when he installs another one) or operates in a non transparent way.
Ads displayed by adwares can be more or less intrusive. An adware may display contextual sponsored links when words bought by advertisers are present in the editorial content of visited websites. An adware can also cause the display of other advertising elements (banners, pop-ups, etc.).
Instead of displaying ads, some adwares generate revenues by overwriting affiliate links (see parasiteware).
See also madware for mobile devices.