Spyware as a category overlaps with adware - generally speaking, the more unethical forms of adware often get dubbed "spyware".
Malware uses spyware for explicitly illegal purposes. The way in which software installs itself or operates without the user's
knowledge or informed consent -- forms the key defining characteristic of spyware.
Data-collecting programs installed with the user's knowledge do not, properly speaking, constitute spyware, provided the user
fully understands who collects what data. Likewise, intrusive adware (of the sort that delivers unrequested advertising pop-ups,
for instance) may not properly constitute spyware, provided the user knows of its installation. Some legitimate software, for
example the Opera Browser, incorporates ads - but does not count as spyware.
More broadly, the term spyware often applies to a wide range of related malware products which do not constitute spyware in the
strict sense. These products perform many different functions, including harvesting private information, re-routing page requests
to illegally claim commercial site referral fees, and installing stealth dialers.
Keeping Passwords Secure
Internet Security Basics
Spyware and viruses
The Danger of Spyware