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Improved cylender is wide open, no choke at all, it will shoot anything, but is best at shooting slugs, and Buck shot. Think about a home defence shootgun, short barrel, IC. It won't hold bird shot to a tight pattern for anything past 35 yds.
Modified choke is between IC, and Full. It's also good with just about anything, but has a tendency to squeeze slugs, and that causes them to not shoot so straight. Works well with bird shot at longer ranges, out to around 45yards, and better, but won't hold the pattern that full will give you. It's good for dove, quail, squirrels, and other things.
Full choke is considered the best for bird hunting, like ducks, and geese, where you will probably be shooting the longest distances. You can shoot slugs, and buck shot thru a full choke, but it is not recomended.