Adding a JAR to the Java Build Path in Eclipse will allow you to use the JAR in Eclipse (eg. the app will be able to use the classes in the JAR).
In this example. Sample.jar has been added to the Java Build Path.
If you want the JAR to be placed in your apps /WEB-INF/lib folder when the WAR is compiled, you'll want to add a Java Build Path Entry. In this way, when the WAR is deployed to your application server, such as JBoss, Tomcat, or WebSphere, the JAR will be in the /WEB-INF/lib directory of your app, and you app will be able to use the JAR.
Now, the JAR will contain "Publish/export dependency: /WEB-INF/lib.
Adding the JAR to the /WEB-INF/lib folder usually only makes sense when no other apps will be using the JAR. When two or more apps need to use the JAR, then you'll usually want to take a different approach.