DRAM or Dynamic RAM, is a type of RAM that only holds its data if it's constantly accessed by the "refresh circuit." By the way in which the cells are constructed, the reading action refreshes the contents of the memory. This refreshing action is why it's called Dynamic RAM. Conventional DRAM chips are not used in new computer systems any more; they have been replaced with such types as DDR RAM and SDRAM.
FPM DRAM, Fast Page Mode, is a bit faster than conventional DRAM, which nonetheless is slow. FPM works by sending the row address once for many accesses to memory in locations near each other, improving access time. Although most PCs still accept FPM chips, no one in their right mind would purchase such a low-end product. It's only suitable for memory buses that operate under the rate of 66MHz. Don't bother with FPM unless you already have it installed, and if you have it installed..UPGRADE!
EDO DRAM, Extended Data Out DRAM, was the most popular form of DRAM technology. It's a bit faster than the FPM DRAM due to the tweak in which the memory access works. In reality, EDO DRAM and FPM DRAM are too similar to bother with. Both were extremely popular in their time, but their time was over in a heartbeat, especially with EDO DRAM. EDO is not suitable for any bus rates over 75MHz just like FPM is not suitable for any bus rates over 66MHz. EDO offers just a 10% increase in speed over FPM, which is too little to waste your time with today. Again, don't waste your valuable money with EDO, save it for something incredible, something amazing, such as SDRAM!
SDRAM, Synchronous DRAM, is more than likely what you have in your computer right now. Introduced in 1997 originally for 66MHz bus speeds, SDRAM has soared with popularity over the years. SDRAM is designed to synchronize itself with the timing of the CPU. The memory controller knows the exact clock cycle when the requested data will be ready, making sure the CPU doesn't have to wait between memory accesses. SDRAM also has interleaving and burst modes which makes retrieval even faster than it already is. The most common bus speeds available for SDRAM are 66, 100, and 133 MHz. Although they have 150MHz chips available, you really don't have a need for a chip with that high of a speed..at least yet.
DDR SDRAM, Double Data Rate SDRAM, is the newest generation of memory chip available. The name explains the beauty of this baby, double the speed! DDR SDRAM functions the same as SDRAM, but doubles the bandwidth of the memory by transferring data twice per cycle! 100 MHz chips turn into 200MHz, 133 MHz goes up to 266MHz, and so on. Now you can easily find chipsets that support these memory chips which operate at 200MHz and 266MHz bus speeds. DDR SDRAM is more expensive then SDRAM, but what do you expect? Double the speed and no increase on value? I didn't think so. DDR SDRAM will be the popular choice soon enough, might as well get a jump-start, right?
RAM can be an awfully confusing aspect of the PC. You must know which one to use; otherwise you might be getting the backend of your potential speed. Lets face it, FPM and EDO are out of the game entirely because no one needs them anymore. We had our fun with them, but they can't handle today's system requirements.
SDRAM is great! With support for bus speeds of up to 150MHz, SDRAM will be in household computers for a while, and the users will be satisfied. But like the generation we are in now, technologies rise too fast to comprehend, and DDR SDRAM will quickly be next in line for mass popularity. Enjoy SDRAM technologies for now as it will shortly be time to move towards a newer RAM technology and leave SDRAM long forgotten.