Adware is software that is freely available for download, but differs from freeware in that it is ad-sponsored. In many cases, adware can be a type of Internet application that runs while the user is online, providing a live feed of changing advertisements, versus built-in static or limited advertising. It's a close cousin of spyware, as many programs collect information about the user to send back to advertisers for purposes of profiling for targeted ad-serving.
In the early days of the dot com boom, advertisers overestimated the response consumers would have to aggressive online advertising, and many dot coms that relied on income from advertising sponsors went bust. In the height of the advertising fury, a few Internet Service Providers (ISPs) even offered free Internet service in return for using adware that ran banners and feeds across every page. When it became clear that people did not want to sit through aggressive advertising, even to get something for free, most ISPs switched to pay services, dropping the advertising interface.
A programmer might choose to create adware in order to collect advertising fees to offset the cost of keeping a program in active development. It is generally viewed unfavorably among the Internet community, however, and many of today’s most popular anti-spyware programs search and alert the user of installed adware. The software is considered a security risk, and Internet users are advised not to install it.
In some cases, a piece of software will be available in a trial version that is fully functional, but it will include ads if the computer user does not pay for the software by the conclusion of the trial period. If the user pays for the software, then all features will be enabled without advertisements. Such programs may or may not collect profiling information.
To keep a computer safe, users should use a good spyware scanner in addition to a virus scanner.