Lello,
Electronic flash is quite a large subject, but with the wide range of flash guns on the market both new and used it does help to understand a little of the development history.
MANUAL : The most basic units have only one output level and have a calculator that when ASA/ISO is dialed in, the appropriate f setting can be read off from the distance scale. Most units from the late 60's / early 70's are of this type.
AUTO : A development from the manual unit, in that greater control of exposure was available automatically, provided the subject was reasonably within the range of the auto setting chosen. Thyristors enabled very fast switching and by adding a sensor to the flash gun to read reflected light, the flash output could be quenched when exposure was deemed by the sensor to be correct. Far less accurate distance calculation was needed so this was a considerable advance on manual. You still needed to set ISO and read off the scale and set lens aperture though. Basic units had 1 auto setting and others like the Vivitar 283 had 4 auto settings.
TTL : With the advent of electronics in camera bodies camera manufacturers came up with TTL. In TTL a sensor in the camera body recorded correct exposure usually off the film and signalled for the flash to be quenched. This allowed for accurate exposure with extension tubes, filters etc and was more accurate than auto.
DEDICATED : Most camera manufacturers introduced dedicated units. These units were specific to certain camera's within the product range, and not suited to use on other manufacturers cameras. Units were generally TTL but not always. Lower end units retained a sensor on the flash. Dedicted flash overcame the problem of forgetting to set appropriate shutter speed in auto or program modes. Turn the flash on and the electronics recognised it and set sync speed.
By now anyone with a compact camera could be excused for laughing, and saying I just point and shoot no problem. How come SLRs can't.
Next stage of development is probably the result of manufacturers luring those same people into the purchase of SLR, lenses etc. Simple flash use for family snaps was an essential nut to be cracked. Advances in the electronics that could be built into cameras and lenses was the way forward.
I can now only speak of Nikon as I have no knowlege of other systems.
As part of its 3D matrix metering Nikon added distance communication between body and lens with the ' D ' series lenses. Introduced to go with the D1 series and D100 cameras Nikon brought out ' DTTL ' flash with two flash units SB50DX and SB80-DX. When you turn the flash on the camera recognises the flash unit and sets appropriate shutter speed, and passes back to the flash unit the camera ISO and lens distance setting. Zoom the lens and the flash unit automatically zooms in the case of SB80-DX. As the camera knows the subject distance and the flash output power in use, it selects the appropriate aperture. So now the SLR flash system as far as point, shoot, and forget has exceeded that of a compact camera due to the output power of flash units available.
Clever as it is, DTTL only survived for a couple of years, and was superceeded by the superior ITTL, and the SB80-DX and SB50-DX were replaced by the SB-800 and SB-600 respectively. At this point Nikon added wireless remote unit for macro to which SB-800 and/or SB600 can be added, with full automatic control of exposure as the wireless system communicates between the camera meter and the individual units in use.
All this modern kit is capable of an incredibly wide range of options/settings between press and forget, and manual. Instruction book for the SB80-DX is 105 pages and my Metz 45CTL if I could find it might run to 10 pages.
Now for the Nikon D80. Nikon system flash guns that will give ITTL ( the latest all singing and dancing totally auto control ) are SB-800 and SB-600. SB-600 has a lower guide number and a few limitations in modes other than ITTL.
Non ITTL on D80 but allowing more options : SB80-DX, SB-28DX, SB-28, SB-26, SB-25, SB-24
Fewer options than above : SB-30, SB22s, SB-22, SB-20, SB16B, SB15
Fewer options still : SB50-DX, SB-23, SB-29, SB21-B, SB-27, SB-29s
I have not mentioned the independant manufacturers units as I do not know where they sit in the hierarchy listed above.
As for remote lead, Foxy sent me a pic of his SC-28 to compare with my SC-17. The difference appears to be that the SC-28 has a locking pin when attached to the camera, and the lead exits to one side, whereas SC-17 has no lock and the lead exits from the front.
On the subject of trigger voltage there does not appear to be a specific max stated by Nikon, so I checked on Nikons own flash guns. From the SB-15 ( 1980's ) onwards all seem to 5V or less with one exception SB-50DX reaching 6V. Make your own mind up from this but it does suggest 6V is safe max.
Don