Hey SCH,
Sh2nu has aldready explained about directional tires, and guptaravi2k's post further explains them.
Just thought i would also mention that there are tyres which are asymetrical (unlike most tyres which have a symmetric tread pattern.)
On the assymetrical tyres, they have an indication on the sidewall for which is the outside (facing away from the car) sidewall and which is the inside sidewall.
These tires usually have more support and a different tread pattern on the outer edge (which takes most of the load during cornering), while the rest of the tire deals with comfort, aquatread etc etc.
Notice that even though asymetrical tires have treading on them that may APPEAR to be directional, they are actually not.
If you had a Asymetrical & directional tire, it means you would have to buy a different set of 2 left tyres and 2 right tyres! lol....i dont think asymetrical&directional tyres exist....anyone?
cya
R