Adware

The term "adware" is an abbreviation for "advertising software" and is used for programs designed to display usually unwanted advertisements to the user. They can be very disruptive, like pop-up windows covering most of the screen and interfering with the user's work, or pretty subtle, like adding small pictures to the sites viewed by the browser. Although sometimes displaying advertisements is a legitimate source of income for the author of an otherwise free product, in most cases the advertisements are unwanted and considered disruptive by the users.