Programs belonging to this class display advertising, usually in the form of banners. They also redirect search requests to advertising sites. Apart from displaying advertising, there is usually no other visible sign that adware is present on the host system.

Adware can be installed to the host system in two ways:

by freeware and shareware which contains advertising components;
when the user visits a web-site which is 'infected' with adware; the adware will then be installed to the host system without the user's explicit consent
The majority of freeware / shareware programs will stop displaying advertisements once they have been purchased and registered. These programs often include integrated adware from third-party vendors. As a rule, such adware remains on the host machine even after the program with which it was installed on the host machine has been registered. In addition to this, if the adware is still being used by a program in order to display advertising, deleting the adware can cause the program in question to crash or malfunction.

If the user visits a 'infected' web-site, the adware will usually be installed to the host machine using hacking techniques, either by exploiting loopholes in the browser's security, or by using Trojan programs (Trojan-Dropper, Trojan-Downloader) designed to install other programs without the user's knowledge or consent. Adware programs which act in this way are often called 'Browser Hijackers'.