actually he asked about "external" flash, which does not necessarily mean off the shoe. I take "external" to mean a flash that is not built into the camera body.
so, let me say that: the camera body does communicate to the flash some parameter information, e.g. how much flash power, what zoom distance, etc. this is assuming that the flash is capable of understanding the camera's commands under the various exposure schemes (e.g. E-TTL, i-TTL, etc).
Now these commands can be transmitted to the external flash via the hotshoe contacts (if the flash is mounted on the shoe). I assume these signals are digital and encoded with data. if the flash is off the shoe and operating wirelessly, then the camera must send these info to the flash either by encoded light pulses through the built-in flash, or via a dedicated wireless transmitter.
not all flash are capable of being wireless or even slave triggered. this depends on the features and spec of the flash itself.