Adware

The most common type of spy you encounter what is known as adware. Adware is a software application in which advertising banners are displayed while the program is running.

These programs include additional code that creates the ads that appear as pop-up windows or through a bar or panel that appears on a computer screen.
The justification for adware is that it helps recover programming period
development costs for some shareware or freeware program that is downloaded to your computer, in other words, it helps to reduce the cost for the user.
Adware is sometimes criticized for including code that tracks a user's personal data and send it on to third parties without your permission or knowledge.
Adware is evidence of violation of privacy and sometimes borders on theft of personal information.