Adware is short for advertising-supported software. Programs displaying advertising material fall under this category. Adware applications will often automatically open a new pop-up window containing advertisements in an Internet browser, or change its homepage. Adware is often bundled with freeware programs, allowing the freeware developers to cover development costs of their (usually useful) applications.
Adware itself is not dangerous – users will only be bothered with advertisements. Its danger lies in the fact that adware may also perform tracking functions (as spyware also does).

If you decide to use a freeware product, please pay particular attention to the installation program. Most installers will notify you of the installation of an extra adware program. Often you will be allowed to cancel it and install the program without adware. However, in some cases programs will not install without adware, or their functionality will be limited. This means that adware may often access the system in a "legal” way, because users have agreed to it. In this case, it is better to be safe than sorry.

If there is a file detected as adware on your computer, it is advisable to delete it, since there is a high possibility that it contains malicious code.