YUI is a comprehensive JavaScript and CSS library for building rich web applications. It provides a wide range of low-level DOM utilities and high-level widgets. Some key features include the Global YUI object, Node utility for DOM manipulation, Event handling, and pre-built widgets like Autocomplete, Calendar and Carousel. YUI is free, scalable, fast, robust, and well-documented with many examples. It allows developers to code efficiently without reinventing the wheel.