Those are standard application archive files,
Since they are a standard any application server (at least in theory) will know how to unpack and deploy them so that clients can access your application. Makes it easy to deploy your application if it is any one of these files WAR or EAR
WAR is for web,
JAR is standard for packaging your classes and it's resources.
ear is for
J2ee application archive which can have jar's as well as war's in them.
WEB-INF again is a standard where you place your application specific 3rd party jars in WEB-INF/lib nad WEB-INF/classes will have your application classes.
JAR, WAR, EAR or kind of zip files where everything you need is zipped up
Hope that helps
Cheers,
Parthi