CD into the root directory of your web app (the one that contains the WEB-INF directory) Type:
This will create a war file in the current directory.
Why create them? Because they are a very easy, clean, and dependable way to ship and deploy an entire web application as a single file.
If you have a running version of Tomcat on your machine (5x or higher), you can see how easy it is to install war files by going to my site and following the directions for any of the demo apps there. It takes between 3 to 5 clicks to install each of them.