"java.lang.NoClassDefFoundError "Error message

Could someone please me out with regards to the below shown error messages? I successfully compiled a java program called "HelloWorld.java" but got the error messages below when i tried to run/execute the java program in question. How could i rectify this problem? I look forward to hearing from anyone wishing to help.

java.lang.NoClassDefFoundError: HelloWorld

Exception in thread "main"

[408 byte] By [olua] at [2007-10-1 20:43:56]
# 1
check your classpath. It should include the directory containing the root of the package tree in which your class resides.
jwentinga at 2007-7-13 2:42:29 > top of Java-index,Developer Tools,Java Compiler...
# 2
jwenting,Thanks a lot for the hint. I really appreciate it.
olua at 2007-7-13 2:42:29 > top of Java-index,Developer Tools,Java Compiler...
# 3
Hi,I also encounter this problem. Can u pls guide me cos i'm don't really understand how u all solve the problem. What Should i change in the classpath... ThankS! Pls help
TofuYana at 2007-7-13 2:42:29 > top of Java-index,Developer Tools,Java Compiler...