Focal Plane Shutter

One of two types of shutter for a camera (see also, Leaf Shutter).

The Focal Plane Shutter is located directly in front of the film plane inside the camera body. Two interlocking curtains move across the film plane and in the process create a slit that exposes one portion of the film at a time. Only at the very middle of the curtains' transit is the film entirely exposed.

This last element is important for the purposes of flash synch. The moment when the flash fires must be timed not to the duration of the exposure, but rather to the point in time when the two curtains are entirely open - thus flash synch for a Focal Plane Shutter tends to be much slower than the shutter's fastest possible speed. To this end, the smaller the shutters have to travel less distance and thus have faster synch speeds. Most modern 35mm cameras with Focal Plane Shutters tend to have flash synch in the 1/125th - 1/250th range. Most Medium Format varieties are limited to 1/30th.

Focal plane shutter curtains travel either vertically or horizontally. Their maximum speed can exceed 1/10000 of a second, while the maximum exposure is virtually boundless.
One should not forget that generally an electronic focal planes shutter drains power when open, which means that a really long exposure can run your batteries dry.
The curtains can be made of different materials, including metal and cloth: cloth curtains are less noisy.

The above discussion of flash synch has been made more complex by the introduction of FP mode in some camera/flash combinations.