CodeIgniter is an open-source web application framework for building dynamic websites using the PHP programming language. It is designed to help developers create applications quickly and efficiently by providing a set of libraries and tools that simplify common tasks, such as database management and form validation. One of the key features of CodeIgniter is its lightweight structure, which allows for fast performance and easy deployment. The framework follows the Model-View-Controller (MVC) architectural pattern, promoting a clear separation of concerns in application development. This makes it easier for developers to manage and maintain their code over time.