PhoneGap is an open source framework that allows developers to build mobile apps using HTML, CSS, and JavaScript instead of native development languages. It uses a webview to contain the HTML and exposes native device APIs like the camera, contacts, and accelerometer through JavaScript. This allows developers to build cross-platform mobile apps that can be deployed to various app stores. PhoneGap has been used to create thousands of apps and supports building apps for many mobile platforms through a single codebase.