From Flash Photography 101:
Flash fact #6*: Every SLR camera with a mechanical shutter has a maximum flash sync shutter speed (1/200 or 1/250 on current Canon DSLRs). This has to do with the way focal plane shutters work. At slower shutter speeds, the first curtain opens, the flash fires, and after the specified time duration, the second curtain closes behind it. At shutter speeds faster than flash sync, the second curtain begins to close before the first curtain is completely open. The second curtain follows the first across the frame, exposing only a slice of the image at any given moment. Firing a flash during this process would illuminate only part of the image.
High speed sync (aka FP flash) produces many low-powered flashes at a very high frequency throughout the duration of the exposure. It is essentially a continuous light source that lasts a very short time, whereas "normal" flash begins after the first shutter curtain opens and ends before the second curtain begins to close.
The disadvantage to FP flash is that it is less efficient and will reduce the effective range of the flash unit. For example, normal flash at 1/200 shutter speed and f/8 will have about 1 stop (1.4 times) more effective range than FP flash at 1/400 shutter speed and f/5.6.
