pentax - What's the difference between vertical and horizontal shutters?

I've always thought that shutters were composed of two vertical curtains that moved along the sensor in order to expose it (at least for most DSLRs). Recently I've bought a Pentax K1000 that surprisingly its curtains move horizontally instead in a vertical motion.

So, this raised a couple of questions:

1. What's the difference between those systems?



2. Is there any advantage in the vertical system over the horizontal system?


3. How does this affect very high speed photography?


4. Does a DSLR with a horizontal shutter exist?