A web application vulnerability is a security flaw in a web application that can be exploited by attackers to gain unauthorized access or perform malicious actions. These vulnerabilities can arise from coding errors, misconfigurations, or inadequate security measures. Common types include SQL injection, cross-site scripting (XSS), and cross-site request forgery (CSRF). When a web application is vulnerable, it can lead to data breaches, loss of sensitive information, and damage to the application's reputation. Organizations must regularly test their applications for vulnerabilities and implement security best practices to protect against potential threats and ensure user safety.