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.