A long pull lever (i.e. for V-brakes, mountain mechanical discs) pulls the cable about twice as long (but about half as hard) as a short pull lever (caliper, cantilever, road discs).
This is determined by the distance between where the cable ends and where the lever pivots. According to this thread, its significantly lower for short pull than for long pull with some variation (it seems like under 30 mm is a cutoff for short pull to long pull if you're picking a lever out of the bin). Park Tool states that the distance from cable end to lever pivot is around 21 mm for short pull (around 42 for long pull lever), and with a 20 degree movement (think the cable end to lever pivot distance as a radius of a circle - with this radius, and the angle at which the lever bottoms out, you can calculate the total possible cable pulled), you pull about 7 mm of cable for short pull (versus 15 mm for a long pull lever).
Obviously, there is some flexibility in this parameter, allowing different manufacturers to design different cable pulls to get different sorts of lever feels. This can also be adjusted on the fly with systems like Avid's Single Digit line of brake levers. Someone claims the adjustment on the Avid Single Digit levers is enough to run short pull (YMMV). Usually when you buy a brake (or lever) from Shimano or SRAM, the manual specifies an "optimal" lever (or brake) to pair with.
If you run long pull lever with short pull brakes, it will be easy to pull the brake, but very hard to apply braking force with a hard lever feel. If you run short pull lever with long pull brakes, you will likely bottom out the lever before stopping (or lock up the wheel) with a soft lever feel. So, make sure to match the pull of the lever with the brake for your safety (obey the manual, except at your own risk!).
You can switch between the systems with a travel agent. Some levers also have a switch which can work with both types with the flick of the switch (Shimano ST-EF65 shifters+brake levers, for example).