Flask is a lightweight WSGI web application framework. It is designed to make getting started quick and easy, with the ability to scale up to complex applications. Flask offers simplicity, flexibility, and extensibility, allowing developers to choose the components they need and structure their applications according to their preferences. Flask's core features include URL routing, template rendering, request handling, session management, and support for HTTP methods. It also provides extensions for adding functionality such as database integration, authentication, RESTful APIs, and more. Flask follows the philosophy of 'micro-frameworks,' emphasizing minimalism and simplicity while providing the essentials for building web applications. Flask is used by developers to build a wide range of web applications, from simple prototypes and personal projects to large-scale enterprise applications.