What does NoClassDefFoundError mean?

[Bill White]

I'm trying to get HelloWorldApp to work using Kaffe.  I compile it
with jikes, and run "kaffe HelloWorldApp.class".  I get an exception
NoClassDefFound error.  I don't have any envariables set.  I have
  o a fairly standard, Debian potato system
  o kaffe version 1.0.5 (the latest stable Debian release) and 
  o the latest stable Debian jikes (1.10).

Just for the record, I have gcj installed as well.  When I compile,
I use "jikes -classpath /usr/share/kaffe/Klasses.jar HelloWorldApp.java"
Apparently jikes doesn't come with a classes.zip file.

TIA.

P.S. I'm really interested in pocketlinux, but I thought I should get
some practice in a less hostile environment first.