A Adware program is one that displays advertisements on your computer or within the program itself. Just because a program is Adware, though, does not make it malicious in nature. In fact there are many legitimate programs, including games, that now display ads on your computer or within the software itself. These types of programs display these ads to generate further revenue for the developers or to promote other software that they may sell. One advantage of a legitimate Adware program is that you can sometimes download the software for free. Instead of the developers charging for the software they will display advertisements within them to cover the costs of development and to generate revenue they would normally get from selling the product. If you then wish to no longer see the advertisements, but would like to continue using the program, you can typically pay a registration fee to the developer. All of these legitimate types of Adware programs will contain an End User License Agreement that will explicitly state if and how advertisements will be shown through the software. When you uninstall these types of Adware, the program will be completely removed and will cease displaying advertisements on your computer.
On the other hand, there are Adware programs that are considered malware or Potentially Unwanted Programs (PUP). These are programs that display advertisements on your computer without your permission or the knowledge of what program is generating them. They are also designed to make it harder to uninstall so that they can continue earning revenue through their advertisements.
Malware Adware are computer infections that are typically installed on your computer through two methods. The first method is when these Adware programs pretend to be something innocuous so that you will download and install them, but once installed all they do is display ads. The other method is when they are installed without your permission or knowledge through Windows or software vulnerabilities on your computer. Adware of this type are the most difficult to remove and typically use protection mechanisms that make it hard to run security programs to assist in removing them.
Adware that are classified as PUPs are typically bundled within other free programs that you download from the Internet. When you install the main program, the adware programs will be installed as well and will display advertisements on your computer. These programs will also not clearly delineate in the End User License Agreement how or when advertisements will be displayed.