OK, from the site mentioned above;
The differential has three jobs:
- To aim the engine power at the wheels
- To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
- To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)
The numbers often quoted is the diff gear ratio (see point 2 above). For example a 3.45:1 ratio will allow the wheels to turn once for each 3.45 turns of the driveshaft which is also the engine speed if your gearbox is in a 1:1 ratio gear (which is 3rd gear in your 4-speed auto or 4th gear in most 5 speed manuals).
As mentioned above you can find out your diff ration by having a crawl underneath - it will be stamped onto a small tag attached to the diff. Depending on what car you have it will probably be around 3:1, say 3.08 or 3.23 for a Falcon.
A shorter diff ratio (larger number, eg 3.45) will allow you car to accelerate faster but will run out of breath quicker and will cruise at higher revs for a given speed.
A taller diff ratio (smaller number, eg. 2.72) will allow for lower revs at a given speed, potentially a higher top speed and better economy. Though there are plenty of other factors at work also.
Diff ratios may need changing if your engine is modified to change the power/torque delivery characteristics, you have substantially different tyre diameters fitted or you are not happy with the factory selected compromise between acceleration/economy/cruising revs.