Answer
Signature files are files that contain definitions of all known viruses. When GMS Anti-Virus runs against an incoming message, it checks the contents of the message, and any attachments associated with the message, against the signature files looking for matches. If any matches are found the message or attachment is deemed to contain a virus and the acion you have set for messages containing viruses is called.
In addition to looking for pattern matches against the signature files GMS Anti-Virus also employs Heuristic techniques to look for indicators of potential viruses and acts on those as well. This additional protection can potentially block viruses before anyone realises they are viruses.
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Keywords:virus scanning virus scan anti virus anti-virus antivirus GMS