Web Application Firewall

Description
A web application firewall (WAF) is an appliance, server plugin, or filter that applies a set of rules to an HTTP conversation. Generally, these rules cover common attacks such as cross-site scripting (XSS) and SQL injection. By customizing the rules to your application, many attacks can be identified and blocked. The effort to perform this customization can be significant and needs to be maintained as the application is modified.

A far more detailed description is available at Wikipedia.

OWASP Projects

 * The OWASP ModSecurity CRS Project's goal is to provide an easily "pluggable" set of generic attack detection rules that provide a base level of protection for any web application.
 * Consider the Web Application Firewall Evaluation Criteria Project (WAFEC) to help evaluate commercial and open source web application firewalls.