Web application

A web application is a software application that runs on a remote server. In most cases, Web browsers are used to access Web applications, over a network, such as the internet. Some web applicatioons are used in intranets, in companies and schools, for example. Web applications are different from other applications because they do not need ot be installed.

Some example web applications are: Facebook (social networking), Flickr (photo sharing), Mibbit (chatting), and Wikipedia.

Web applications are popular because most computer operating systems have web browsers. Programmers can easily change a web application. Users do not need to install any new software to see these changes.

Benefits and drawbacks

Benefits

Web applications run "inside" a browser; no complex installation is needed.
Web applications require very little disk space (or computing power) on the client. All the client does is display the data.
Web applications solve some of the "compatibility issues" (Windows, Mac, Linux); all that is needed is a browser.

Drawbacks

Because they run inside a web browser, most web applications "look" very different to regular programs. The user experience is different, usability/ease of use may be different as well.
Web applications need to be coded so they follow standards. In this case, any browser that also follows the standard can be used. Small changes in the browser implementation of a standard may mean that a given browser can no longer be used to access a web application
Web applications need a connection to the server where the application runs, all the time. In addition to the connection itself, they may need a certain bandwidth. When the connection is lost, or the bandwidth is not sufficient, the application may not be usable; in the worst case, data may be lost.
Many applications are dependent on the server that hosts them. When the server is switched off, or the company goes bust, the application is no longer usable. Traditional applications continue to work.
The company offering the web application has complete control over it. This also means that they may launch a new version when they want to; the option to "skip" a less popular version does not exist.
In many cases, the data is stored remotely too. It may not be possible to export the data so that it can be used with another application.
The company can theoretically track anything the users do. This can cause privacy problems.

Other websites

HTML 5 Draft recommendation, changes to HTML and related APIs to ease authoring of web-based applications.
The Other Road Ahead – An article arguing that the future lies on the server, not rich interfaces on the client
Web Applications at the Open Directory Project
Web Applications Working Group at the World Wide Web Consortium (W3C)