Advertising Spyware
Stealth advertising components that are installed by some "shareware" products (and sometimes, legitimately purchased commercial software) and may collect personal information from your computer. These "adbots" are usually tied to a dodgy shareware program you have installed.

Spyware
Stealth components and background processes that may violate your privacy or expose your computer to attack.

"Backdoor Santas"
Non-stealth "freeware" and shareware apps that may transmit personal information or expose your computer to attack, under the pretense of providing a useful service.

Homepage Hijackers
Once one of these nasty ad-trojans worms onto your system, it will constantly reset your homepage (and maybe Search, etc.) to where they want you to go. You can't change it back!

Foistware (Everything-installs-it-can't-get-rid-of-it)
Unwanted application programs that come along, trojan-style, with completely unrelated software. Usually because some jerk is getting paid to foist it on your system whether you want it or not. Since they tag along with so many different pieces of third-party software, it is not uncommon to get re-infected with these foistware products again and again

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Trojan Horses
Programs for the specific purpose of violating your privacy, stealing data, taking over or trashing your computer.