The quick answer is as such:
0.00" constriction = Cylinder
0.05" constriction = Skeet
0.10" constriction = Improved Cylinder
0.20" constriction = Modified
0.30" constriction = Improved Modified
0.40" constriction = Full
The 'constriction' is the amount of reduction in inside diameter at the choke. To figure this out you'll need to know what the bore diameter is of the gun you're using. This used to be a standard, but now manufacturers have changed that. You'll need to do some research or get the right tool to figure out the bore of your gun. And to throw another wrench in the gears, the contriction chart is a rule of thumb, the actual choke is determined by 'patterning' your gun at 40 yards. You can find this information online as well.
There is a lot to this, so I tried to keep the answer relatively short. Again, you can find a lot of good info online.