See Neil van Niekerk’s excellent tutorial he has just posted demonstrating High Speed Sync flash (HSS) on a Nikon D3 and on a Canon 5D.
For Olympus and Panasonic users, this technology (which was actually first developed for Olympus OM film SLR cameras) is equivalent to Super FP mode.
The actual maximum flash sync speed in normal flash mode depends upon the camera – most Micro Four Thirds and entry level dSLR cameras have a flash sync of ~1/160th-1/180th sec, while pro level dSLRs tend to have a flash sync of 1/250th sec.
High Speed Sync (HSS or Super FP mode) is very confusing to most people, and this tutorial goes a long way to helping you understand it.
It’s main purpose is to allow wider apertures to be used in flash photography in sunlit conditions, but as Neil points out, most single flash units will not be able to overpower full sun even in HSS mode as HSS mode inevitably results in a substantially reduced maximum power output or Guide Number, and this reduction is power is generally in proportion to the higher the shutter speed selected.