Java compiler compiles the Java source code files (*.java) into binaries files (*.class). You would use the Java de-compiler to convert java class files into source code files (*.java).
Java de-compiler is very useful especially if you have *.class files and you do not have access to the source code. Some vendors do not ship the source code for java class files, in which case you use the java decompiler to look at the source code.
See also the "javap" command from Sun Microsystems. This command is available as soon as you install the JDK from Sun Microsystems. At unix prompt type -
bash$ javap -help bash$ javap -c <filename>.class
The following are the list of Java decompilers available -