A web application or web app is a program that runs in a web browser, it is created with the web standard programming languages JavaScript, HTML and CSS and is not developed for just one specific platform. Web apps compete against native apps to seamlessly integrate the latest characteristics and features for mobile such as Geolocation, Application Cache, Motion Sensors, Touch Events, etc.
The app can be anything from a simple, real-time messaging service to a complex game or image editor. The apps can be designed for desktop browsers, mobile browsers, or both!
External Links: Mobile HTML5 | Web Application | Standards for Web Applications on Mobile: Current State and Roadmap | Google Web Fundamentals