RDRAM is Rambus Dynamic Random Access Memory.  It has a unique technology developed by Rambus, Inc.  These modules, also called RIMM, are extremely fast and use a narrow, high-bandwidth channel to transmit data at a much higher speed than SDRAM.  Motherboards are specifically designed to accept RIMMs so these modules cannot be mixed with SDRAM DIMMs.

You can easily identify if a module is a RIMM if it has an aluminum covering placed over the RAM chips.  This sheath acts as a heat sink to protect the chips from overheating.