What is Adware?

Legal Definition
Adware, or advertising-supported software, is any software package that automatically renders advertisements in order to generate revenue for its author. The advertisements may be in the user interface of the software or on a screen presented to the user during the installation process. The functions may be designed to analyze which Internet sites the user visits and to present advertising pertinent to the types of goods or services featured there. The term is sometimes used to refer to software that displays unwanted advertisements known as malware.
-- Wikipedia
Legal Definition
Software added to other software to display ads. It is a threat to the user and leads to hacking. It is installed without the users knowledge. Software has been made to prevent this.