Adware is.....

Software that may have been installed on your computer by a remote site. Many free utilities that you download from the Internet will install hidden software that sends details of the websites you visit and other information from your computer (which can include your email address) to advertisers so they can target you with popup ads and spam. See Spam, Spyware.

While not necessarily malware, adware is considered to go beyond the reasonable advertising that one might expect from freeware or shareware. Typically a separate program that is installed at the same time as a shareware or similar program, adware will usually continue to generate advertising even when the user is not running the originally desired program. See also cookies, spyware, and web bugs.