Web applications have basically three layers to their model.  The first layer is on the users side and consists of a basic browser.  The second layer contains a dynamic content generation tool -- such as Java, active server pages, or php.  The third layer is where the data is stored and consists of back end database software. 

The figure below illustrates each of these levels.

From http://www.acunetix.com/websitesecurity/web-applications.htm

Sometimes these layers are refered to as:

• Presentation
• Application
• Storage

The user's side with the browser is also referred to as the client, or thin client since all that is needed is a web browser.  Aslo -- web applications are referred to as web widgets.

Types of Web Applications