There are whole books written about chokes and the subject is so big no post is going to more then scratch the surface.
Basically there are three types of shotgun chokes.
The "fixed" choke is the oldest, and this is a constriction (or choke) built into the barrel at the muzzle. These chokes cannot be changed without major modifications of the barrel.
The adjustable choke, is a device attached to the muzzle, usually by brazing it on.
These often include a muzzle brake in the design.
To tighten or open up the choke, the device is screwed in or out. Inside the choke, spring "fingers" open or close to increase or open up the constriction.
One of the most common is the old Cutts compensation/choke device.
The "screw-in" choke is the newest. These are screwed into the muzzle, and come in various chokes.
To change the degree of choke, you simple remove one and install the other.
How much choke there is, and how it effects how tight the pattern is, depends on the specific gun, the ammunition, and the choke used.
Since each gun/ammo/choke shoots differently, most shotgunners spend a lot of time shooting test patterns at patterning boards.
This allows the shooter to select the ammo and choke that produces the BEST patterns for that specific combination.
A combination that shoots excellent patterns for one person will often shoot poor patterns in another gun even though it's the exact same make and model, with the same ammo and choke.
Selecting a gun/ammo/choke combination that shoots the best patterns seems to be more an "art" than a science, and what another gun/shooter is using is absolutely no guarantee that it will shoot well for you.
It's entirely possible for one gun with a more open choke to shooter better patterns than another gun with a tighter choke, thus the need to pattern the gun/ammo to determine exactly how the gun/choke/ammo is actually performing.
