An application server is a software framework that provides an environment for running applications, typically web-based. It acts as a mediator between the user interface and the backend database, handling business logic and processing requests. This allows developers to focus on creating applications without worrying about the underlying infrastructure. Application servers support various programming languages and frameworks, making them versatile for different types of applications. They often include features like security, transaction management, and scalability, which are essential for maintaining performance and reliability in environments where multiple users access the application simultaneously.