http://en.wikipedia.org/wiki/Adware
"Some adware is also shareware, and so the word may be used as term of distinction to differentiate between types of shareware software. What differentiates adware from other shareware is that it is primarily advertising-supported. Users may also be given the option to pay for a "registered" or "licensed" copy to do away with the advertisements."
"It is not uncommon for people to confuse "adware" with "spyware" and "malware", especially since these concepts overlap. For example, if one user installs "adware" on a computer, and consents to a tracking feature, the "adware" becomes "spyware" when another user visits that computer, and interacts with and is tracked by the "adware" without their consent."