The original design of the SCSI standard allowed two different types of buses, the single ended and differential. The cable uses twisted pair wiring, this makes it less prone to electrical interference and to picking up noise. This means that differential SCSI needs more wires and more electronics, and consequently also means higher prices, so as you guest most users systems use single ended SCSI because their cheaper and work good in most applications, like small systems and where electrical noise is not a problem. Since differential SCSI is less prone to electical interference the cable lengths are greatly improved. You can use any connector on the cable and in any order.
External devices use a different cable and connector for obvious reasons.
The cable is usually consisted of 25 twisted pair wires, so you get a total of 50 wires. This means that you need a cable for each additional external device. The standard SCSI connector has 50 pins in two rows and is termed as the type A connector from the SCSI specifications, this connector looks like a Centronics printer connector yet has 50 pins instead of 36. Some devices use a regular 25 pin D-shell connector, this connector is good, but can only handle single ended SCSI because they don't have enough connections. You can click here to view the 25 pin D-shell connector and it's pin assignment .