Anti-Spyware software is similar to an antivirus. It is designed to detect and remove malware that your antivirus software might not detect, and can also offer protective methods from malware that may try to compromise or infect your system.
An anti-spyware solution cannot be a replacement for your antivirus software!
Anti-Spyware solutions sometimes use real-time protection (similar to that of an antivirus) in order to detect malicious behaviour. Despite this, anti-spyware solutions usually only allow detection and removal of malware existing on the user’s computer.
In order to detect existing malware, anti-spyware solutions use very thorough scanning techniques that can detect further malicious coding or behaviour.
However, what anti-spyware may have in terms of malware detection, it usually lacks in prevention from further infection.
It is recommended that you always have an anti-spyware installed on your computer or at hand to tackle malware that might not be detected by normal protective measures.