Here is a nice explanation of memory that I found at Crucial.com.
Memory modules 101: The basics
Module Size
The total number of bytes (usually expressed in megabytes or MB) on the memory module.
Package
The number of pins (30, 72, 100, 144, 168, 200) and the type of memory module (SIMM, SODIMM, DIMM, RIMM) for this part number.
Feature
The type of DRAM on this memory module. Current DRAM types includes FPM, EDO, SDRAM, DDR, and RDRAM.
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Configuration
This describes the structure of the memory module. The first number is the depth of the module (or, the number of signals the module sends at one time). The second is the width (of the module's data bus). To get the total megabytes of the module, multiply the depth by the width and divide by 8 (or 9 for ECC or parity; if you're not sure if you have ECC/Parity divide the width by which ever one gives you an integer). For example, a 16M72 is 16 x 72 / 9 = 128MB.
DIMM Type
This refers to how the DIMM processes signals. There are three DIMM types: buffered, registered, and unbuffered. Most memory modules are unbuffered.
Buffered modules contain a buffer (usually when there's a lot of memory) to help the chip set cope with the large electrical load required. The buffer electrically isolates the memory from the controller to minimize the load that the chipset sees.
Registered modules contain a register that delays all information transferred to the module by one clock cycle. Again, this is usually done on modules with a lot of memory to help ensure that the data is properly handled.
Most buffered and registered modules also have ECC and are used in servers. FPM and EDO modules are either unbuffered or buffered. SDRAM modules are either unbuffered or registered.
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Error Checking
There are two different types of error checking: parity and ECC (Error Checking and Correction) Parity modules have an extra chip that detects if data was correctly read or written by the memory module, depending on the type of error. However, it will not correct the error.
ECC modules have an extra chip that detects if the data was correctly read or written by the memory module. If the data wasn't properly written, the extra chip will correct it in many cases (depending on what type of error). Non-ECC (also called non-parity) modules do not have this error-detecting feature.
EDO and FPM modules utilize either parity or ECC. SDRAM, DDR, and RDRAM modules utilize ECC. All types of memory can also be non-parity.
Component Speed
This is how fast the components on your memory module are capable of running. Speed is expressed in nanoseconds (lower is faster). Keep in mind, with SDRAM, the memory module will only run as fast as the speed of your processor's front side bus.
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Voltage
This refers to the amount of power the memory module needs to function. The module gets its power from your system's power supply, which is why designers are always trying to reduce memory voltage requirements. FPM and some EDO parts run at 5 volts, some EDO and all SDRAM parts run at 3.3 volts, and DDR runs at 2.5 volts.
SDRAM Timings
This refers to the latency of the parts, or the amount of time it takes for the memory to respond to a command. Latency is measured in terms of clock cycles and is often noted as CL2 (two clock cycles) or CL3 (three clock cycles). For most applications there is very little difference in performance between CL2 and CL3 parts.