Spyware
Spyware, close relative to adware, consists of programs, cookies, and registry entries that track users' activites and send this data to others. Spyware is usually used for marketing purposes, so that companies can better cater to their customers. Spyware is nearly always installed secretly, hence the name spyware. Most feel that spyware violates their privacy, and spyware can definitely harm a computer's performance. Excessive spyware can hog system resources, thereby slowing down your computer. It can also slow an internet connection, as usage data is sent.
Spyware is an inevitable byproduct of using the internet. Fortunately, there are easy ways to combat it. Check out the anti-spyware software section!
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 spyware?" Tech FAQ. Accessed April 15, 2007. <http://www.tech-faq.com/spyware.shtml>