Adware

Adware is any software application in which advertising banners are displayed while the program is running. The authors of these applications include additional code that delivers the ads, which can be viewed through pop-up windows or through a bar that appears on a computer screen. The justification for adware is that it helps recover programming development cost and helps to hold down the cost for the user.
Adware has been criticized because it usually includes code that tracks a user's personal information and passes it on to third parties, without the user's authorization or knowledge.

There are basically two types of adware: the first is a legitimate software that is free and is supported with advertising. Users typically install this software knowing that they will be subjected to ads when they use it (Examples include Eudora, Gmail, Hotmail, and other free programs).

The other type of adware is illegitimate programs that install without the users knowledge and display ads without the users consent. This type of adware is considered a malware because it was installed without the user’s authorization and can cause problems due to use of resources and network bandwidth.