I'm not sure you're getting the answers you need, though it's all here amongst the posts above. Likewise, we still don't know why you need this information and therefore how best to answer. So an overview as biefly as I can...
All DSLRs use a focal plane type shutter (google) which means they can only sync with conventional flash up to a certain shutter speed, around 1/200sec to 1/250sec. If you fire the flash at higher speeds than this, only part of the picture will be exposed by the flash and the rest of the frame will be black.
However, the flash pulse itself, the flash duration, is very short - typically 1/1000sec on full power reducing to maybe 1/30,000sec at lowest power. So even with relatively long shutter speeds you can still have a very fast effective flash exposure time.
To enable you to sync flash at much higher focal plane shutter speeds, as you might need to do in bright ambient light with fill-in flash, most decent flash guns have a high speed sync mode (Nikon call it FP flash). As described above, the gun pulses extemely rapidly, firing repeated flashes at up to 50,000 times per second, and it keeps this up just long enough for the focal plane shutter to complete its full cycle, thus exposing the sensor evenly all over with no black bits.
The downside of this trick is that as the power of the flash gun is spread over a much longer time, the brightness is diluted, and in addition a lot of this light is wasted as it will inevitably fall on part-closed shutter curtains, which is obviously doing nothing. What this means in practice is that total flash power in HSS mode is substantially reduced, so maximum flash range is cut quite a lot (and the recycle times are long as the flash is always working flat out). And the faster the shutter speed, the more light is wasted and the worse it gets. It's still a fantastically useful feature though, especially when it is fully integrated into the auto-TTL metering system (Canon E-TTL, Nikon iTTL).
The best Canon gun is the 580EXII (Nikon SB900 is similar) which is powerful and does all these things, and more. It is also a master controller for multi-remote off camera flash set ups.