An application which is accessed via web browser over a network is called web application or web based application. All the websites are examples of web applications.

Web applications are quickly replacing traditional client server softwares. Client-Server computing requires separate software to be installed on the client machine. Any update in server software may also require update at client end. This means that client needed some preparation before software can be run. On the other hand a web application works without any installation at client end. The output of web application is usually HTML / XHTML which is rendered by a web browser.

Structure of web application
Web application is written in a server side scripting language like PHP. It resides on a web server which is accessible by simply typing URL in address bar of web browser. When user types address of website for example www.programming-web.com, browser transmits request to the web server which hosts the required site. On web server, web server software like apache receives this request and processes it. Processing may involve complex process such as database operations. After processing, web sever provides output which is then passed to requesting client. The output generated by web server includes only those scripts which can be rendered by web browser. So no special software is need at client end. Web browser receives this response which is rendered by browser and displayed to user. Above process repeats when user perfumes an action which requires communication with server such as submitting entry form or viewing another page.