This category covers all applications which send private information without user consent/awareness. They use tracking functions to send various statistical data such as a list of visited websites, email addresses from the user's contact list, or a list of typed keystrokes.

The authors of spyware claim that these techniques aim to find out more about users’ needs and interests and allow better-targeted advertisement. The problem is that there is no clear distinction between useful and malicious applications and no one can be sure that the retrieved information will not be misused. The data obtained by spyware applications may contain security codes, PINs, bank account numbers, etc. Spyware is often bundled with free versions of a program by its author in order to generate revenue or to offer an incentive for purchasing the software. Often, users are informed of the presence of spyware during a program's installation to give them an incentive to upgrade to a paid version without it.

Examples of well-known freeware products which come bundled with spyware are client applications of P2P (peer-to-peer) networks. Spyfalcon or Spy Sheriff (and many more) belong to a specific spyware subcategory – they appear to be antispyware programs, but in fact they are spyware programs themselves.
If there is a file detected as spyware on your computer, it is advisable to delete it, since it most likely contains malicious code.