Flask is a micro web framework written in Python. It is designed to be lightweight, modular, and easy to extend, providing the essential tools needed to build web applications without imposing any dependencies or project structure. Flask follows the "micro" framework philosophy, meaning it provides the basics and lets developers choose additional libraries and tools as needed. This flexibility makes Flask ideal for both small applications and large projects, where customization and control over the application's components are important. Flask is known for its simplicity, readability, and ease of use, making it a popular choice for web developers.