User Interface Libraries are collections of pre-written code that help developers create user interfaces more efficiently. These libraries provide reusable components, such as buttons, forms, and navigation menus, which can be easily integrated into web applications. By using these libraries, developers can save time and ensure consistency in design across their projects. Popular User Interface Libraries, like React and Vue.js, allow for the rapid development of interactive user experiences. They often come with built-in features that enhance usability and accessibility, making it easier for developers to focus on functionality rather than starting from scratch. This leads to better user engagement and satisfaction.