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.