Java IDL (Interface Definition Language) provides a binding between Java objects and the OMG's (Object Management Group's) CORBA. CORBA, the common object request broker, allows objects written in different languages such as C++, Smalltalk, and Java, to communicate and interact across distributed, heterogeneous systems. Java IDL includes an IDL-to-Java compiler and a lightweight ORB that supports IIOP (the Internet Inter-Orb Protocol). This is now included as a standard part of Java 1.2.
For an introduction to CORBA and Java and how they relate, see:
by Robert Orfali & Dan Harkey
This book has stirred a lot of controversy among the distributed object community. The consensus seems to be that it does a good job with CORBA and IDL, but not so great with RMI and DCOM. However this particular community isn't one known for rational, unbiased, objective discussions in the first place so that's no great surprise. In any case, whether you love it or hate it you should read it.