Before I get into all the standards, I need to explain how speeds are derived.  In the original SCSI, only one byte (8 bits) could be moved down the bus at any given time.  The frequency of the bus (Hertz) was 5MHz, so therefore the maximum transfer rate was 5MB (mega bytes) per second.  The next evolution was Fast/10 SCSI, moving data at a blistering 10MHz, for a transfer rate of 10MB/s.  Added to this was an improvement called Wide.

What this did was literally widen the bus from 1 byte wide to 2 bytes.  Now, at a frequency of 10MHz, 20MB could be transferred!  Because there were two types, wide and 'regular', the regular width was called Narrow.As you may know, there are two types of SCSI cables in use right now, 50 pin and 68 pin.  They may come in internal or external versions, but they're basically the same.  A Wide bus always goes over a 68 pin cable, while a Narrow bus can go over either kind of cable, although people usually have it going over a 50 pin cable because of the cost.  What Wide does is widen (duh) the bus to 2 bytes (that's why you need the extra wires), instead of just one byte.  If your bus is two bytes wide, guess what?  Now you can transfer two bytes at once, doubling your speed!  The byte that the Narrow bus uses is called the Low byte.  The second byte that the Wide bus can use is called the "High" byte.  

The next big improvement was Fast/20.  This is known as Ultra SCSI.  this, paired with a wide bus, allowed for transfers up to 40MB/s (20MHz * 2 bytes).  Next up was Fast/40 (Ultra 2), with wide throughput up to 80MB/s.  Ultra3 brought wide throughput of 160MB/s!  Note that both Ultra2 and Ultra3 devices operate at 40MHz.  Ultra3 has a few tricks up its sleeve to let it transfer at 160MB/s.

Instead of re-hashing tables that are already written I'm going to provide a link to the SCSI Trade organization's Terms and Terminology page.  Keep the window open as a reference.

To start explaining the table let's start by examining the three interfaces, Single-Ended (SE), Low Voltage Differential (LVD), and High Voltage Differential (HVD).  SE is the original interface.  It is limited in that as the speed (MHz-wise) of the bus goes up, the allowable length goes down because of signal degradation over longer distances.  HVD was the next big improvement, allowing for lengths up to 25 meters.  However, HVD devices are very expensive.  LVD is the latest interface, allowing buses up to 12 meters long.  It works in the same way as HVD, just in a better fashion.  Although I know how they work, I feel that it's beyond the scope of this website.  If you really have a hard on for that type of stuff, you don't need me to tell you how it works; go get a book or something.

HVD and SE are electrically incompatible, that is, you can't string HVD and SE devices together.  LVD and SE devices are compatible, but the system can only support LVD if and only if every device is an LVD device.  If just one SE device is inserted, the entire bus will revert back to the limitations of SE.

The referred table had a maximum bus length for each standard, and it is very important to keep to that length.

This table didn't show it, but currently there are 3 versions of SCSI.  Each version brought new improvements; all of the above standards are distributed throughout the 3 versions.  They are different versions in the sense that a committee sat down and defined (and redefined for SCSI-2 and SCSI-3) the latest and greatest in SCSI.  Here they are along with what they offer.

SCSI-1

• 5MB/s max transfer rate
• Transfer rates of about 1MB/s or less.
• Only SE interface

From today's standpoint SCSI-1 seems pretty crappy but at the time is was competing against systems with 300KB/s transfer rates.

SCSI-2

• Fast/10
• Wide
• Added HVD interface
• More intelligent interface
• Improved compatibility

SCSI-2 was/is a pretty good interface.  Transfer rates zoomed to 20MB/s, and it was much easier to use.

SCSI-3

• Fast/20 (Ultra1)
• Ultra (Wide)
• Fast/40 (Ultra2)
  The LVD interface was introduced with Ultra2 SCSI.
• Ultra2 (Wide)
• Ultra3 (Note that after Ultra2, all new speeds are wide only)
  One way to think of Ultra3 is like Ultra2 with six optional features tacked on.  These features are:
  1. Fast-80
  2. CRC
  3. Domain Validation
  4. Packetized SCSI
  5. QA
  6. Double Transition Clocking
• Ultra 160
  This is a unique implementation of Ultra3 that was developed by an industry group, including Adaptec.  It includes Fast-80, CRC and Domain Validation.  Check the jargon section for info on these terms.
• Parallel or Serial interface
• With Serial interface:
  • Faster performance
  • Networkable
  • Bus length in kilometers, not just meters
  • Almost unlimited number of devices
  • More reliable
  • Many media types, such as optical fiber, copper coaxial cable, twisted pair, and wireless
• The Serial interface probably won't be compatible with today's devices, but that's why the parallel version was created.

With SCSI-3, all SCSI devices were split into two categories: Parallel and Serial.  All the "normal" devices we've come to know and love fall into the parallel category.  Serial is obviously the more advanced of the two, as you can see.  The serial interface uses an protocol called "Fiber Channel".  This protocol specifies a transfer rate from 133Mb/s to 16Gb/s (that's bits per second).  However, today's most common rate is about 1Gb/s, for a sustained net data throughput of 100MB/s.  When SCSI-3 Serial was defined, three carriers were considered, Fiber Channel, FireWire (1394), and the Serial Storage Architecture.  Fiber Channel won out because it was the best performer and it was the most versatile.  It's a very versatile protocol that can be used for much more than SCSI setups.  For example, it can be used for a computer network as ethernet would.  Also, it can be used on more than one type of cabling.  Optical fiber, copper coaxial, and twisted pair cabling can be used.

Granted, this is a lot of information to digest, but that's what you get when you have a system (SCSI, that is) that has evolved so much and kept up with the times.

Now that we have a fairly good grasp of the standards, let's take a look at terminating the bus.