How to un-archive a .jar archive?

Hi:
I have downloaded a .jar archive file. According to the instruction, I need to use the "jar" command-line utility to un-archive the distrubution first. Does anyone know how to do it? Thanks.

Two sites with instructional information:
1. How To Open A Jar
2. Using JAR files: The Basics
Good Luck.

Well, Dirk, the first one isn't really about JAR files

Short answer: jar xf filename.jar

Also, remember that JAR simply uses the ZIP format -- so you can also use a program like Winzip.