Comes in both 30 pin and 72 pin sizes.
Fast Page Mode DRAM
Fast Page Mode DRAM is plain old DRAM as we once knew it. The problem with standard DRAM was that it maxes out at about 50 MHz.
EDO DRAM gave people up to 5% system performance increase over DRAM. EDO DRAM is like FPM DRAM with some cache built into the chip. Like FPM DRAM, EDO DRAM maxes out at about 50 MHz.
SDRAM (Synchronous DRAM)
Almost all systems used to ship with 3.3 volt, 168-pin SDRAM DIMMs. SDRAM is not an extension of older EDO DRAM but a new type of DRAM altogether. SDRAM started out running at 66 MHz, while older fast page mode DRAM and EDO max out at 50 MHz. SDRAM is able to scale to 133 MHz (PC133) officially, and unofficially up to 180MHz or higher.
DDR (Double Data Rate SDRAM)
DDR basically doubles the rate of data transfer of standard SDRAM by transferring data on the up and down tick of a clock cycle. DDR memory operating at 333MHz actually operates at 166MHz * 2 (aka PC333 / PC2700) or 133MHz*2 (PC266 / PC2100). DDR is a 2.5 volt technology that uses 184 pins in its DIMMs. It is incompatible with SDRAM physically, but uses a similar parallel bus, making it easier to implement than RDRAM, which is a different technology.
DDR2 SDRAM or double-data-rate two synchronous dynamic random access memory, is a part of the SDRAM (synchronous dynamic random access memory) family of technologies. It is an evolutionary improvement over its predecessor, DDR SDRAM. It comes in varients of DDR2-400, DDR2-533, DDR2-667, DDR2-800, and DDR2-1066. It uses 240-pins.
DDR3 SDRAM or double-data-rate three synchronous dynamic random access memory, it is an evolutionary improvement over its predecessor, DDR2 SDRAM. It comes as DDR3-800, DDR3-1066, DDR3-1333 and DDR3-1600 speeds and also uses a 240pin socket. However it’s notch location is different to that of DDR2.
Rambus DRAM (RDRAM – RIMM)
RDRAM is a serial memory technology that arrived in three flavors, PC600, PC700, and PC800. PC800 RDRAM has double the maximum throughput of old PC100 SDRAM, but a higher latency. RDRAM designs with multiple channels, such as those in Pentium 4 motherboard.
SODIMM’s are generally used in notebooks, they come in the many variants listed above. Note that the notch on the memory tends to be in a different spot based on the different types.