this stands for Rambus inline memory (RDRAM), this is latest incarnation of system memory and is capable of running at 800mhz compared to the 66/100/133 or so standards currently At the moment it can be expensive when compared and is mainly suitable for high demanding applications The Rambus technology is used in the Pentium 4 based systems
Rambus In-line Memory Module: This is the form factor for Rambus RDRAM By comparison, SDRAM is mainly found on DIMMs, and EDO RAM is usually on SIMMs RIMMs require that if you do not fill all RIMM slots with RDRAM memory, you must keep the empty slots filled with termination boards to ensure that the high speed Rambus memory signals do not bounce improperly