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.