A Potentially Unwanted Program, or PUP, is a piece of software that you agree to install on your computer but, for most people, doesn't provide any useful service. These types of programs are typically bundled with free software that you download from the Internet. When installing the free programs you will also be prompted to install these other programs (PUPs) as well. In many cases the information about these programs being installed will be hidden in lengthy End User License Agreements or in confusing installation scripts. PUPs are bundled with a developer's free software because the developer generates revenue for each of these programs that are installed on computer. In fact, there are some developers that create small free utilities just so they can distribute them with PUPs in order to earn money.
Once installed, many of these programs can be difficult to remove and become more of a nuisance rather than a benefit. Many of them will display pop up ads, nag screens, or other types of alerts that are designed to convince you to purchase the software or perform some other actions. In some cases PUPs can be more damaging to a computer than traditional malware by causing application freezes, crashes, and other instability.