A unique string of bits, or the binary pattern, of a virus. The virus signature is like a fingerprint in that it can be used to detect and identify specific viruses. Anti-virus software uses the virus signature to scan for the presence of malicious code.
15. Most antiviruses have hundreds of thousands or even millions of malware signatures and yet they scan many files in a reasonable short time with high detection rates. Even real-time scanners doesn't slow computer noticeably but provide strong protection against threats.  YARA is a free tool for “helping malware researchers to identify and classify malware samples.” Like ClamAV, it can scan files using custom signatures, looking for byte sequences and strings; its signature syntax also supports regular expressions and conditionals. The tool’s website provides several sets of signatures that could be used as starting point to learn about creating your own. YARA is a very popular approach to creating custom malware signatures. A project for exchanging YARA signatures is Yara Exchange Group. The problem with most anti-malware tools is that they rely on signatures to detect the malicious code. Whenever a new virus, spyware program or other piece of malware is discovered, the vendor has to update the database that the anti-malware tool uses to recognize the new malware.  Vscan for Custom Malware Signatures. Vscan is a free toolkit for “making fast but crude measurements of the prevalence of named textual features in algorithmically selected samples of large corpora.” In other words, it can scan files to identify those that match user-specified patterns. The tool’s website provides several sets of signatures that could be used as starting point to learn about creating your own. YARA is a very popular approach to creating custom malware signatures. A project for exchanging YARA signatures is Yara Exchange Group. Much of the need to provide signatures varies with the type of scanner it is, i.e. with what that scanner is charged with detecting. For example, adware and spyware are not nearly as prolific as viruses, thus typically an adware/spyware scanner may only provide weekly signature updates (or even less often). Of course, it's simply not practical to release an individual signature for each new virus discovered, thus antivirus vendors tend to release on a set schedule, covering all of the new malware they have encountered during that time frame. viruses. Because of this, the response times providing malware signature updates are longer. While viruses, once released, propagate on their own and survive or die out based on their payloads, malware can be updated and modified after being released by its creator. Antivirus companies can usually update their antivirus signatures within 24 hours if a new virus or varient of an existing virus is discovered. A malware signature update takes approximately 72 hours, if not longer. This time difference is do to various factors. First, a virus can be sent for inspection and analysis A unique string of bits, or the binary pattern, of a virus. The virus signature is like a fingerprint in that it can be used to detect and identify specific viruses. Anti-virus software uses the virus signature to scan for the presence of malicious code You can use it to create custom malware signatures. The tool’s website provides several sets of signatures that could be used as starting point to learn about creating your own. YARA is a very popular approach to creating custom malware signatures. A project for exchanging YARA signatures is Yara Exchange Group. Microsoft Security Essentials protects Windows systems against Trojans, rootkits, worms and other malware. The software, the successor to Windows Live OneCare, acts as a free first line of defense for scores of Windows XP, 7 and Vista users. (The Windows 8 counterpart is called Windows Defender). 
http://security.stackexchange.com/questions/30362/how-do-antiviruses-scan-for-thousands-of-malware-signatures-in-a-short-time https://zeltser.com/custom-signatures-for-malware-scan/ http://www.windowsecurity.com/articles-tutorials/viruses_trojans_malware/Hunt-Down-Kill-Malware-Sysinternals-Tools-Part1.html https://zeltser.com/custom-signatures-for-malware-scan/ http://antivirus.about.com/od/whatisavirus/a/virussignature.htm http://searchsecurity.techtarget.com/answer/Malware-signature-updates http://www.webopedia.com/TERM/V/virus_signature.html https://zeltser.com/custom-signatures-for-malware-scan/ http://www.eweek.com/security/microsoft-to-issue-anti-malware-signatures-after-windows-xp-support-cutoff.html
what are malware signatures