What is spyware?

Definition of spyware (quoted from spywareinfo.com):

"Spyware is software or hardware installed on a computer without the user's knowledge which gathers information about that user for later retrieval by whomever controls the spyware."

Generally, much so-called 'adware' can be considered a form of spyware.

In SpywareGuard, additional programs called 'browser hijackers' will be detected as spyware. These programs may install silently and, while many do gather information or perform tracking, some simply just 'hijack the browser' - or they change various browser settings and make them hard to change back. Some browser hijackers even embed themselves into your system and can cause a loss of internet connectivity if not removed correctly.

SpywareGuard may also detected some "premium-rate dialers". Many times, these dialers are installed without the user's knowledge (and even silently in the background by just viewing a webpage) and are not noticed until the month's phone bill shows a charge to some out-of-country number for hundreds of dollars.