Trail is the difference in where the tire contact patch is, relative to the imaginary line that goes through the centerline of the headset angle. It is calculated mathematically by taking the headset angle, wheel diameter and fork rake. There are plenty of sites that go into detail on the nuts and bolts. What is counter-intuitive about fork rake is that increasing fork rake actually decreases train, all other things remaining equal. There is online someplace a good article I read about a guy who did experiments with trail, trying to find a bike setup that would be unrideable due to negative trail (you can see negative trail on a shopping cart caster; because it's negative is why you see a lot of them oscillating side to side as you push the cart). The results as I recall (keeping in mind that recall is not like a recording) was that even an unstable trail bike can be ridden successfully. You could see how unstable it was by just pushing the bike without rider. The conclusion was that we are able to control a bike, even if it is unstable. This is why some people prefer to say the steering is quicker or slower.
