A web app is an application that uses technologies such as Javascript, CSS, and HTML5 and is executed in a web browser. Developers release their applications as web apps because of the larger flexibility of the format. The application can be run directly from a website, or, if it uses the HTML 5 Offline Application Cache and Web Storage, can be downloaded and installed locally, for offline use.
Reference http://en.wikipedia.org/wiki/Web_app
