Web application is a program that is used on a client computer linked to the Internet network. The use of a browser for the interface tends to popularize new technologies that allow to get from Internet, software as sophisticated as desktop one. Such software is named "rich Internet application"
A desktop application is a program that is not part of the operating system. Unlike script, it must have a user interface. But it depends upon the operating system or the environment, Java or .NET for examples, and it interacts with the hardware through this environment
At the opposite, a Web application is independent of the environment of the client, it is executed on the server and makes use of the server resources through the browser, including content of databases. The user interface is provided by the browser and is made mainly of HTML forms for now.
Web application must be distinguished of Content Management System, that are limited to put a content online, while a Web application has more various tasks. A CMS may work statically to build a content for a website, or dynamically at net-surfer request, in this case it is a Web application.
Web services are systems built to allow interoperability between computers through a network. They can use the SOAP XML format to send requests, and the WSDL format to build an interface that describes the service.
