Spyware is malicious software running on a host that may intercept or take information from the host system without a users consent or knowledge. Spyware is also capable of using a hosts Internet connection without the knowlege or consent of the user, in order to deliver that information to an unauthorized third party.
This software not only uses available bandwidth on a network connection but also consumes system resources to the point of making the host unusable in some cases.
Spyware can be classified into multiple categories depending on the behavior of the software.