A web application firewall (WAF) is a security tool designed to protect web applications by filtering and monitoring HTTP traffic between a user and the application. It helps prevent attacks such as SQL injection, cross-site scripting, and other vulnerabilities by analyzing incoming requests and blocking harmful ones. WAFs can be deployed in various ways, including as hardware appliances, software solutions, or cloud-based services. They work by applying a set of rules to identify and mitigate threats, ensuring that only legitimate traffic reaches the web application, thereby enhancing overall security and performance.