In order to enable computers to work faster, there are several types of memory available today. Within a single computer there is no longer just one type of memory. Because the types of memory relate to speed, it is important to understand the differences when comparing the components of a computer.
SIMM (Single In-line Memory Modules)
SIMMs are used to store a single row of DRAM, EDO or BEDO chips where the module is soldered onto a PCB. One SIMM can contain several chips. When you add more memory to a computer, most likely you are adding a SIMM.
The first SIMMs transferred 8 bits of data at a time and contained 30 pins. When CPU's began to read 32-bit chunks, a wider SIMM was developed and contained 72 pins.
72 pin SIMMS are 3/4" longer than 30 pin SIMMs and have a notch in the lower middle of the PCB. 72 pin SIMMs install at a slight angle.
DIMM (Dual In-line Memory Modules)
DIMMs allow the ability to have two rows of DRAM, EDO or BEDO chips. They are able to contain twice as much memory on the same size circuit board. DIMMs contain 168 pins and transfer data in 64 bit chunks.
DIMMs install straight up and down and have two notches on the bottom of the PCB.
SODIMM (Small Outline DIMM)
SO DIMMs are commonly used in notebooks and are smaller than normal DIMMs. There are two types of SO DIMMs. Either 72 pins and a transfer rate of 32 bits or 144 pins with a transfer rate of 64 bits.
RDRAM - RIMM
Rambus, Inc, in conjunction with Intel has created new technology, Direct RDRAM, to increase the access speed for memory. RIMMs appeared on motherboards sometime during 1999. The in-line memory modules are called RIMMs. They have 184 pins and provide 1.6 GB per second of peak bandwidth in 16 bit chunks. As chip speed gets faster, so does the access to memory and the amount of heat produced. An aluminum sheath, called a heat spreader, covers the module to protect the chips from overheating.
SO RIMM
Similar in appearance to a SODIMM and uses Rambus technology.
Technology
DRAM (Dynamic Random Access Memory)
One of the most common types of computer memory (RAM). It can only hold data for a short period of time and must be refreshed periodically. DRAMs are measured by storage capability and access time.
Storage is rated in megabytes (8 MB, 16 MB, etc).
Access time is rated in nanoseconds (60ns, 70ns, 80ns, etc) and represents the amount of time to save or return information. With a 60ns DRAM, it would require 60 billionths of a second to save or return information. The lower the nanospeed, the faster the memory operates.
DRAM chips require two CPU wait states for each execution.
Can only execute either a read or write operation at one time.
FPM (Fast Page Mode)
At one time, this was the most common and was often just referred to as DRAM. It offered faster access to data located within the same row.
EDO (Extended Data Out)
Newer than DRAM (1995) and requires only one CPU wait state. You can gain a 10 to 15% improvement in performance with EDO memory.
BEDO (Burst Extended Data Out)
A step up from the EDO chips. It requires zero wait states and provides at least another 13 percent increase in performance.
SDRAM (Static RAM) 
Introduced in late 1996, retains memory and does not require refreshing. It synchronizes itself with the timing of the CPU. It also takes advantage of interleaving and burst mode functions. SDRAM is faster and more expensive than DRAM. It comes in speeds of 66, 100, 133, 200, and 266MHz.
DDR SDRAM (Double Data Rate Synchronous DRAM)
Allows transactions on both the rising and falling edges of the clock cycle. It has a bus clock speed of 100MHz and will yield an effective data transfer rate of 200MHz.
Direct Rambus
Extraordinarily fast. By using doubled clocked provides a transfer rate up to 1.6GBs yielding a 800MHz speed over a narrow 16 bit bus.
Cache RAM
This is where SRAM is used for storing information required by the CPU. It is in kilobyte sizes of 128KB, 256KB, etc.
Other Memory Types
VRAM (Video RAM)
VRAM is a video version of FPM and is most often used in video accelerator cards. Because it has two ports, It provides the extra benefit over DRAM of being able to execute simultaneous read/write operations at the same time. One channel is used to refresh the screen and the other manages image changes. VRAM tends to be more expensive.
Flash Memory
This is a solid-state, nonvolatile, rewritable memory that functions like RAM and a hard disk combined. If power is lost, all data remains in memory. Because of its high speed, durability, and low voltage requirements, it is ideal for digital cameras, cell phones, printers, handheld computers, pagers and audio recorders.
Shadow RAM
When your computer starts up (boots), minimal instructions for performing the startup procedures and video controls are stored in ROM (Read Only Memory) in what is commonly called BIOS. ROM executes slowly. Shadow RAM allows for the capability of moving selected parts of the BIOS code from ROM to the faster RAM memory.
