How does anti-spyware software protect users?

It protects users by preventing spyware getting on their technological devices! = )

So, a spyware is a malicious program that installs without user's permission and gathers personal data about the user.

Anti-spyware use an engine, a database (constabtly updated) that contains all informationd about spywares. Then you may want to run a scan (quick or full) of the hard disk or of the registry or memory.