Definitions of adware vary depending on where you look for it. Generally speaking, adware refers to software that is supported by advertising or uses ads in any other form to generate revenue. We have to distinguish between adware that is added to programs as a third party option, for instance as toolbars that users can but do not have to install, programs and services that displays advertisement in the program directly, and malicious uses where adware is installed or displayed without the user's consent.
In legit software programs, adware is often displayed in the form of advertisement that generates revenue for the software developer. Often, two program versions are offered: a free ad-driven version, and a pro version that costs money but does not show ads. Mobile apps follow this practice quite often.
Sometimes, devices ship with an advertisement option as well. The popular Amazon Kindle is available as a version with ads on the homescreen or an ad-free version that costs more.
While it is all about displaying advertisement to users, it often serves other purposes as well. It is quite common that aware records information about a user's PC and its activities to submit those to a central server where they are processed. Profiling can be very lucrative not only for determining the ads that are best to be displayed to the user, but also by selling the information.
A common form of adware that has been plaguing Windows PC users for some time are toolbars and related offers that are added to software installations. The installers are sometimes designed to trick users into installing adware. It can happen that adware is installed on a user's system even if the custom installation option is selected during installation.
It is recommended not to install any third party tools that software installers make available to you during installation as you may end up with a set of system changes that you did not want made in first place. This often includes the installation of a toolbar in browsers that support it, the changing of the home page and search provider.
You can use a program like Avast Browser Cleanup Tool to remove several high profile toolbars from your system, or Adware Cleaner which does the same more or less.
A program that goes deeper than those two is Malwarebytes Anti-Malware. It is a free program for the Windows operating system that may not remove toolbars and the like but adware that is more malicious in nature.
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Ghacks is a technology news blog that was founded in 2005 by Martin Brinkmann. It has since then become one of the most popular tech news sites on the Internet with five authors and regular contributions from freelance writers.