Officially known as Small Computer System Interface, this type of interface has been around for over a decade.
It is used to connect a system (such as a P.C.) to a disk drive, a scanner, or a high-speed printer.
Most of its growth had slowed down and stabilized in the early 1990's, but within the last two years
it has grown again and again, so much so that I was confused with all the revisions.
Here is a chart and some notes I put together to try to demistify all the terms.

SCSI bus is 8 to 32 bits wide.
16-bit and 32-bit variations are called wide-SCSI and call for an additional cable.
Wide-SCSI is a SCSI-2 feature.
Commands, messages, and status are sent across the bus exclusively
using asynchronous transfers.  Max ~3MHz.
Data is sent usually synchronously nowadays.
SCSI-2 allows up to 10MHz.
Wide SCSI requires an additional cable (or additional wires in the current cable).  Its name
applies to either 16-bit or 32-bit transfers.
NAME Speed Bits Pins
Async SCSI 1.5MHz-4MHz 8 50
SCSI-1 5MHz 8 50
SCSI-2 10Mhz 8 50
Fast-SCSI 10MHz 8 50
Wide-SCSI N/A 16 (or 32, but no implementations yet) 68
Fast-Wide 10MHz 16
Wide-Fast20 20MHz 16
Wide-Fast40 40MHz 16
Wide-Fast80 80MHz 16
Ultra-2 (LVD) ? ? 68 or 80-pin SCA

SCA = Single Connector Attachment.  An 80-pin connector popular in Unix workstations where the SCSI pins are combined with power and ground pins into one keyed connector.

Single-ended: Each SCSI data pin either has a signal or is a ground pin.  Popular with PCs.
Differential: Each SCSI data pin either is +signal or -signal.  Popular with Unix servers.  Allows longer cable
distances.  Think large server rooms or numerous attachments.

LVD: Low-voltage differential.  Became very popular with Sun servers and newer RAID servers.
The normal voltage is 5V, but I don't know what voltage the low-voltage is.  Probably 3.3V.   The current
trend is to hang the bytes-per-second on to the word Ultra, i.e. Ultra-160 or Ultra-320.  These terms
HVD: Originally there were only single-ended and differential SCSI.  Since LVD was created differential
SCSI has been nicknamed HVD.  If you see a connector or terminator that only says "differential SCSI" or
has the universal differential SCSI symbol with the horizontal dash sticking outside the diamond, that is HVD.
You can use the same cables for single-ended and differential SCSI, but the terminators are different.
Single-ended terminators won't work properly to terminate differential busses.  I have read that "Ultra2 SCSI"
was available in both HVD and LVD, but no longer in single-ended configuration.  There were a few confusing
years there.  Check, table 1, for an accurate chart.
I've had problems terminating a Sun HVD bus with a non-Sun HVD terminator. After unsuccessful attempts to use a LVD terminator, a forced-perfect terminator, and an HP HVD terminator I finally threw in the towel and EBay'ed a Sun HVD terminator. Those things are expensive and kind of rare ($20 in 2006), but if you need one you don't have much choice.

Last update: 12-Feb-2007