jQuery Mobile is a JavaScript framework for building mobile web applications using HTML markup. It allows developers to build sites and applications that work across desktop, tablet, and smartphone browsers with a unified user interface. Some key features include progressive enhancement to provide functionality on all devices, customizable themes, touch events, and support for common mobile platforms like iOS, Android, and Windows Phone. jQuery Mobile builds on jQuery and jQuery UI to simplify development and provides widgets and functionality out of the box for building mobile interfaces.