Onsen is a front-end UI framework designed for developing mobile web apps. It offers a comprehensive library of components that are optimized for performance and user experience across multiple platforms, including iOS and Android. Onsen supports popular web technologies like HTML, CSS, and JavaScript and is compatible with frameworks such as Angular, React, and Vue.js. The framework provides tools for creating native-like navigation and smooth animations while allowing developers to focus on building high-quality applications. Onsen is ideal for developers looking to create cross-platform mobile applications with a seamless and responsive UI.