"Patch and vulnerability management is a security practice designed to proactively prevent the exploitation of IT vulnerabilities that exist within an organization. The expected result is to reduce the time and money spent dealing with vulnerabilities and exploitation of those vulnerabilities. Proactively managing vulnerabilities of systems will reduce or eliminate the potential for exploitation and involve considerably less time and effort than responding after an exploitation has occurred. Patches are additional pieces of code developed to address problems (commonly called bugs) in software. Patches enable additional functionality or address security flaws within a program. Vulnerabilities are flaws that can be exploited by a malicious entity to gain greater access or privileges than it is authorized to have on a computer system. Not all vulnerabilities have related patches; thus, system administrators must not only be aware of applicable vulnerabilities and available patches, but also other methods of remediation (e.g., device or network configuration changes, employee training) that limit the exposure of systems to vulnerabilities. This document provides guidance on creating a security patch and vulnerability management program and testing the effectiveness of that program."
NIST SP 800-40; NIST Special Publication 800-40
NIST Computer Security Resource Center: http://csrc.ncsl.nist.gov/publications/drafts.html