What do we mean by "Web Application"? There is an incredible range of sophistication in web applications from a simple company web site with "brochure ware" to sites like Yahoo or Amazon with complex search engines and order fulfillment. One way to look at the web application architecture is to take the model of a traditional business transaction application and to replace the user front end by the web site. A Customer acquires goods and/or services from your company, in exchange for money. There are mechanisms in place to facilitate that transaction between client and company. Instead of a sales rep, a clerk, a cashier, or such person, you have a browser pointing at a web site. The company is never closed! Customers can serve themselves.
The Living Creature - Testing Web Applications