Stands for "Rambus Dynamic Random Access Memory." It is a type of RAM made by Rambus (big surprise) and is the fastest type of computer memory available. Typical SDRAM can transfer data at speeds up to 133 MHz, while standard RDRAM can crank it up over 1 GHz. Though some motherboards can use RDRAM as system memory, it is so fast, most boards cannot fully benefit from the speed. Because of this, RDRAM is typically used for video memory on graphics acclerator cards, for cache memory (located on the CPU), and for system memory in high-performance workstations and servers.
An improvement to RDRAM called Direct Rambus (DRDRAM) allows for even faster data transfer rates. DRDRAM uses a 16-bit bus rather than the 8-bit bus DRAM uses, which means it can handle 8 operations at once and can transfer data at a speeds of 1.6 GHz. Now that's fast.