What is a PUP?
a. A program that monitors keystrokes.
b. A small window that suddenly appears while browsing the Web, which downloads spyware to the user's computer.
c. Software in which advertising banners are displayed while the program is running.
d. A program that may be unwanted, despite the possibility that users consented to download it.
Answer from the SearchSecurity.com glossary:
A PUP (potentially unwanted program) is a program that may be unwanted, despite the possibility that users consented to download it. PUPs include spyware, adware, and dialers, and are often downloaded in conjunction with a program that the user wants. (Read the rest of this definition.)