DRAM
Dynamic random access memory (DRAM) is the most common kind of random
access memory (RAM) for personal computers and workstations. Random
access means that the PC processor can access any part of the memory or
data storage space directly rather than having to proceed sequentially
from some starting place. DRAM is dynamic in that, unlike static RAM
(SRAM), it needs to have its storage cells refreshed or given a new
electronic charge every few milliseconds. Static RAM does not need
refreshing because it operates on the principle of moving current that
is switched in one of two directions rather than a storage cell that
holds a charge in place. Static RAM is generally used for cache memory,
which can be accessed more quickly than DRAM.  DRAM stores each bit in
a storage cell consisting of a capacitor and a transistor. Capacitors
tend to lose their charge rather quickly; thus, the need for recharging.

SDRAM
(synchronous DRAM) is a generic name for various kinds of dynamic
random access memory (DRAM) that are synchronized with the clock speed
that the microprocessor is optimized for. This tends to increase the
number of instructions that the processor can perform in a given time. The
speed of SDRAM is rated in MHz rather than in nanoseconds (ns). This makes
it easier to compare the bus speed and the RAM chip speed. You can convert
the RAM clock speed to nanoseconds by dividing the chip speed into 1
billion ns (which is one second). For example, an 83 MHz RAM would be
equivalent to 12 ns.

DDR SDRAM
(double data rate SDRAM) is synchronous dynamic RAM (SDRAM)
that can theoretically improve memory clock speed to at least 200 MHz*. It
activates output on both the rising and falling edge of the system clock
rather than on just the rising edge, potentially doubling output. It's
expected that a number of Socket 7 chipset makers will support this form
of SDRAM.

Synchronous DRAM speed is measured in MHz rather than nanoseconds. You
can convert the RAM clock speed to nanoseconds by dividing the chip speed
into 1 billion ns (which is one second). For example, an 83 MHz RAM would
be equivalent to 12 ns.

RDRAM / DRDRAM
(Rambus Dynamic Random Access Memory) is a memory subsystem that
promises to transfer up to 1.6 billion bytes per second. The subsystem
consists of the random access memory (RAM), the RAM controller, and the
bus (path) connecting RAM to the microprocessor and devices in the
computer that use it. Direct Rambus (DRDRAM), a technology developed and
licensed by the Rambus Corporation, is the latest version and is expected
to help accelerate the growth of visually intensive interfaces such as
3-D, interactive games, and streaming multimedia. Rambus is intended to
replace the current main memory technology of dynamic random access memory
(DRAM). Much faster data transfer rates from attached devices such as
videocams using FireWire and the Accelerated Graphics Port (AGP) make it
important to reduce the bottleneck in getting data into the computer,
staging it in RAM, and moving it throught the microprocessor and to the
display or other output devices.

Direct Rambus (DRDRAM) provides a two-byte (16 bit) bus rather than DRAM's
8-bit bus. At a RAM speed of 800 megahertz (800 million cycles per
second), the peak data transfer rate is 1.6 billion bytes per second.
Direct Rambus uses pipelining to move data from RAM to cache memory levels
that are closer to the microprocessor or display. Up to eight operations
may be underway at the same time. Rambus is designed to fit into existing
motherboard standards. The components that are inserted into motherboard
connections are called Rambus in-line memory modules (RIMMs). They can
replace conventional dual in-line memory module.

SIMM
A SIMM (single in-line memory module) is a module containing one or
several random access memory (RAM) chips on a small circuit board with
PINs that connect to the computer motherboard. Since the more RAM your
computer has, the less frequently it will need to access your secondary
storage (for example, hard disk or CD-ROM), PC owners sometimes expand
RAM by installing additional SIMMs. SIMMs typically come with a 32 data
bit (36 bits counting parity bits) path to the computer that requires a
72-pin connector. SIMMs usually come in memory chip multiples of four
megabytes.

The memory chips on a SIMM are typically dynamic RAM (DRAM) chips. An
improved form of RAM called Synchronous DRAM (SDRAM) can also be used.
Since SDRAM provides a 64 data bit path, it requires at least two SIMMs
or a dual in-line memory module (DIMM).

DIMM
A DIMM (dual in-line memory module) is a double SIMM (single in-line
memory module). Like a SIMM, it's a module containing one or several
random access memory (RAM) chips on a small circuit board with pins
that connect it to the computer motherboard. A SIMM typically has a 32
data bit (36 bits counting parity bits) path to the computer that
requires a 72-pin connector. For synchronous dynamic RAM (SDRAM) chips,
which have a 64 data bit connection to the computer, SIMMs must be
installed in in-line pairs (since each supports a 32 bit path). A
single DIMM can be used instead. A DIMM has a 168-pin connector and
supports 64-bit data transfer.

RIMM
A memory module developed by Kingston Technology Corp. that takes up
less space inside the computer than the older DIMM module and has
different PIN characteristics. A RIMM has a 184-pin connector and an
SO-RIMM module has a 160-pin connector. An SO-RIMM is smaller and is
used in systems that require smaller form factors. While RIMM is
commonly believed to stand for "Rambus inline memory module," Kingston
Technology has trademarked "RIMM" and uses only that term.

SMI
Synchronous Memory Interface.