Adware
According to Wikipedia, "Adware or advertising-supported software is any software package which automatically plays, displays, or downloads advertisements to a computer after the software is installed on it or while the application is being used. Some types of adware are also spyware and can be classified as privacy-invasive software."
Application
Advertising functions are integrated into or bundled with the software, which is often designed to note what Internet sites the user visits and to present advertising pertinent to the types of goods or services featured there. Adware is usually seen by the developer as a way to recover development costs, and in some cases it may allow the software to be provided to the user free of charge or at a reduced price. The income derived from presenting advertisements to the user may allow or motivate the developer to continue to develop, maintain and upgrade the software product. Conversely, the advertisements may be seen by the user as interruptions or annoyances, or as distractions from the task at hand.
Some adware is also shareware, and so the word may be used as term of distinction to differentiate between types of shareware software. What differentiates adware from other shareware is that it is primarily advertising-supported. Users may also be given the option to pay for a "registered" or "licensed" copy to do away with the advertisements.
Programs have been developed to detect, quarantine, and remove spyware. As there are many examples of adware software that are also spyware or malware, many of these detection programs have been developed to detect, quarantine, and remove adware as well. Among the more prominent of these applications are Ad-Aware and Spybot - Search & Destroy. These programs are designed specifically for spyware detection and will not detect viruses, although some commercial antivirus software can also detect adware and spyware, or offer a separate spyware detection package.