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The PATH problem causes more difficulties to beginners than almost anything else. What it means is that your OS will go and look for a program called "java" or "javac" and the PATH tells it where to look.
Read
this Common Problems page from the Java Tutorials, particularly the bit about "java is not recognised etc" and where it tells you how to set the PATH permanently. If you are on Windows, the entry will begin with the drive letter (probably C) and end with bin, so a common entry would be "C:\Program Files\Java\jdk1.6.0_06\bin".