Some Minolta and Sony camera bodies feature a PC-sync socket.
It is used to trigger larger studio flashes. These flashes are typically
very powerful, are powered via power lines instead of batteries, have a
modeling light, and their output can be adjusted. They are, however, not
TTL capable. This means that the camera can only trigger these flashes but can not
stop flash output. You have to use these flashes in manual
mode. You have to use a lot of calculations or an external flash exposure meter
to calibrate the flashes.
This is a more powerful but less comfortable system. But when you use these flashes you
(hopefully) know what you're doing.
These are either small battery-powered flashes or large studio flashes which are
triggered by a pulse of light. The idea is that when the main flash on the camera fires
it also triggers the slave flashes. Again, this is a non-TTL system. These flashes
can only be triggered, but not stopped by the camera. You have to adjust the output
from these flashes manually.
You have to be careful to not use wireless flash or pre-flash metering with the
main flash. Otherwise the control pulses or metering flashes trigger the slave flashes,
but the shutter is not yet open at this time. You're in trouble when you use a
digital camera, because these always either use pre-flash metering or ADI with
pre-flash metering for automated flash. You have to switch the flash to
manual mode to avoid the pre-flash. Again, if you use such a system you
have to know what you're doing.