Can you tell me something about web applications?

It's not clear what exactly you wish to know about web applications. But in general, a 'web application' is a name given to software applications (programs) that can be accessed over the Internet, mostly through the web browser.

CrazyEngineers, for example, has a collection of web applications dedicated to engineers from all over the world. 

You will have to think about an application that can be accessed over the Internet. For example, bus reservation system is an online application. Here's a sample list of web applications-

1. Train reservation system
2. Online attendance system
3. Online blogging system
4. Online articles management system
5. Wiki systems
6. Online library management system
7. Online profile builder system
8. Simple web search engine
9. Online quizzing system
10. Online personal contacts management system

I hope this helps.