WAR file is a "Web Application Archive" -- it's comprised of all the pieces of your web application: JSPs,
Servlets, Images, Static HTML, images, etc. -- and all packaged in a single snazzy little file.
Couple great advantages of a WAR file -->
1- Easily transferred from development to production machines (there aren't TONS of files to move and make sure they're all in the right order).
2- They're self describing -- it contains a deployment descriptor (web.xml) that tells the app server exactly what's in it and how its run.
3- It's standards based -- so you can (in theory) deploy this one WAR file onto ANY app server Vendor (HP, BEA, IBM etc).