Adware
Adware is very similar to spyware and as a term, is often grouped with spyware in common use. There are some basic differences, however. While spyware entails spying on computer usage and activities, adware is software that is used to display advertisements. Like spyware, adware can slow down your computer by utilizing RAM (random access memory), and also slows your internet connection by retrieving advertisements to display on your screen.
Unlike most other malware, adware isn't necessarily bad. Good adware will ask for permission before installing itself, or will consist of unobstrusive advertisements in applications. Bad adware, however, gets in the way of day-to-day activities and serves as a nuisance.
It is important to note that removing adware may cause shareware programs to cease functioning. This is because some shareware creators include adware with their software. That said, adware is usually safe to remove, and is usually worth removing because of the otherwise negative effects it has on your system.
Resources
Baratz, Adam & McLaughlin, Charles. "Malware: what it is and how to prevent it." Ars Technia. November 11, 2004. <http://arstechnica.com/articles/paedia/malware.ars/>
"What is adware?" Tech FAQ. Accessed April 15, 2007. <http://www.tech-faq.com/adware.shtml>