Going from most open to tightest, here's the progression with the most commonly accepted amount of contriction in thousandths....
Cylinder (0)
Skeet (5)
Improved cylinder (10)
Light Modified (15)
Modified (20)
Improved Modified (25)
Light Full (30)
Full (35)
Extra Full (40)
Too darn tight(anything above 40)
Naturally, some differ on just what constitutes a given choke. Back before choke tubes, we got along fine with 3 degrees of choke, IC, Mod, and Full. But since so many of us are willing to spend lots of money trying to find a gadget that will make up for our less than perfect shooting, choke degrees are proliferating like Bacilli. Many of the upscale choke makers list 10 sizes.
And actually, choke is determined by the percentage of pellets with a 30" pattern at 40 yards by the classic standard. We use Points Of Constriction as a shorthand and easy method of reference.
Slugs tend to work best through the chokes more open than Modified.
HTH...