Learning this type of programming technique requires you to have a backgeound on the following:

1. AJAX

2. XML

3. HTML

4. CSS (optional) 

 
BUT WHY DEVELOP WEB APPLICATIONS? 

Web applications have more advantages compared to desktop applications. First, web applications could be easily deployed. Second, it has many supports and easy to develop. Lastly, Web Apps are more flexible and more portable since you can access it through any web browsers.

Although desktop applications out fits web applications in terms of user-friendlyness and richness, with the use of AJAX, it would answer all these problems and even left behind desktop applications.