(n.) Adware is software that contains advertisements embedded in the application. Adware is considered a legitimate alternative offered to consumers who do not wish to pay for software. There are many ad-supported programs, games or utilities that are distributed as adware (or freeware).
Today we have a growing number of software developers who offer their goods as “sponsored software” until you pay to register. This is the most common form of adware. If you’re using legitimate adware, when you stop running the software, the ads should disappear, and you always have the option of disabling the ads by purchasing a software license key.
Adware as a Form of Spyware
Another use of the phrase adware is to describe a form of spyware that collects information about the user in order to display advertisements in the Web browser. Unfortunately, some applications that contain adware track your Internet surfing habits in order to serve ads related to you. When the adware becomes intrusive like this, then we move it into the spyware category and it then becomes something you should avoid for privacy and security reasons.
Recommended Reading: The Difference Between Adware & Spyware.