This week's book giveaway is in the Testing forum. We're giving away four copies of The Way of the Web Tester: A Beginner's Guide to Automating Tests and have Jonathan Rasmusson on-line! See this thread for details.
The classpath for a web app is (at least)
plus all the JARs in the WEB-INF/lib/
In addition, servers may add further directories to the classpath to give access to shared resources between web apps and provid system / server resources.