Cylinder? Full? Improved Modified? Shotgun choke tubes can be confusing, so Brownells Gun Techs™ Steve Ostrem and Caleb Savant explain some choke tube basics for us. From narrowest to most open, the common chokes are Full, Improved Modified, Modified, Improved Cylinder, and Cylinder. There are others, like specialized choke tubes for hunting turkey, but these are the closest to standardized constrictions. There is no Official Choke Tube Authority that sets standards, so tubes tend to be manufacturer-specific. Read the specs to find out the exact constriction of a particular choke tube.
Waterfowl hunters have to use steel shot by law. Steel shot tends to pattern tighter than lead shot. With steel you typically want a choke that's one step more OPEN to get a pattern similar to what you would with lead shot. Example: If you're shooting an Improved Modified choke in lead, you'll want to go with a Modified choke for steel to get a similar-size pattern. But be careful: if you use too tight a choke with steel shot, the intense compression from the wad will actually cause the shot to scatter excessively when it finally clears the gun! This can damage the choke or the barrel itself.
The maker of your shotgun or the choke tubes you're using in it will tell you how tight you can go with steel shot. While we should try to be flexible, Steve warns us about how shotgun barrels can FLEX in a bad way. Hint: avoid shooting steel shot through an old double-barreled shotgun with fixed chokes.
Now, for the deer hunters, what about SLUGS? The basic types of slugs are the (1) sabot slug with a smaller-caliber bullet inside a plastic sleeve, (2) "pumpkin ball" rifled slug with fins that cause it to spin in flight, and (3) the old-style "chunk o' lead" finless slug. Good news: RIFLED choke tubes will work with all 3 types of slugs. With normal choke tubes, you should use only Cylinder or Improved Cylinder with slugs.