Pentium 4 processors and their attendant motherboards come in different versions that support one of four types of memory. These memory types include SDRAM, RDRAM, DDR and DDR2.
SDRAM
SDRAM stands for synchronous dynamic random access memory. The term "synchronous" means the memory times its data transfers to the pace of the computer's internal clock signal. "Dynamic" means the memory cells need constant power to retain information. "Random access" means the memory does not have to be read in sequence.
RDRAM
RDRAM stands for Rambus Direct RAM. Rambus is a specific memory manufacturer. According to PCGuide.com, Rambus works like SDRAM, except it makes two data transfers per clock signal rather than one. It also requires that all motherboard memory slots be filled with either memory or "continuity" modules. The latter adds no memory but only relays signals.
DDR
DDR stands for double data rate. Like Rambus, DDR transfers data twice per clock signal. However, unlike Rambus, DDR is non-proprietary. Thus, it's made by many different manufacturers. DDR does not require all memory slots to be full.
DDR2
According to IXBT labs, DDR2 differs from DDR in that it contains a 4n prefetch buffer. This means that it can send four bits of data per memory access, as opposed to DDR's two.
Tags: clock signal, means memory, data transfers, memory slots