A web app(lication) uses technologies such as javascript, CSS, and HTML5 and is executed in a web browser.

A web application is an application that is accessed by users over a network such as the Internet or an intranet. The term may also mean a computer software application that is coded in a browser-supported programming language (such as JavaScript, combined with a browser-rendered markup language like HTML) and reliant on a common web browser to render the application executable.

Reference http://en.wikipedia.org/wiki/Web_app

More complex web apps that mimic desktop application behaviour in regard to user-interaction, complexity and the dynamic loading of partial pages via AJAX are also called RIAs (Rich Internet Applications).