The camera sliders in Lux are set up just like a real world camera.  Changing the sensitivity is like changing the film speed (faster film is more sensitive to light).  Exposure is shutter speed and tells you how long the film is being exposed to light in fractions of a second.  F-Stop tells you how wide the lens aperture is open.  In that one the numbers are reversed, as the f-stop number gets smaller more light is let onto the film.

F-Stop also plays a big role in DoF, if I remember right smaller f-stops give you deeper areas in focus and larger ones make it much more shallow.  It's been a while though so I'm not 100% sure of that one, besides we've got lots of time to figure out how Lux will handle it yet.

On an unrelated subject, is anyone having exports crashing Lux?  I've got a couple of scenes I was trying that export fine back crash Lux during the loading process.