A
Potentially Unwanted Program,
is a program that does something *unwanted* such as install a million toolbars (AVG I'm looking at you), or sells their browsing data to a company.
This differs from normal spyware or adware in that the user agreed to it in the EULA or
privacy policy that they didn't read before
installing, therefore making it somewhat legal.
Similar to PUPs are grayware.