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.
A JAR file is an archive of Java class files and resources. It is a way of bundling related java code into a single distributable resources rather than needing to send each class separately. You can typically open them in any program that supports ZIP files if you want to look inside.