When I started shooting I was puzzled when people would talk about "shotgun ckokes". What the heck was that? Did they mean that I had to hold it differently, like when you choke up on a baseball bat?
A shotgun doesn't work the way that a rifle or handgun does. Instead of a single bullet that fits snugly in the barrel, a shotgun is designed to fire a shell packed with loose-fitting pellets (click on the ZOOM button to see the pellets).
This means that, instead of a single bullet scraping it's way down the barrel and travelling in one direction with a great deal of accuracy, the pellets in a shotgun exit in a cloud of gas. As soon as it exits the barrel the gas starts to expand, which means that the pellets start to drift apart. So how do you keep them close together to increase the punch downrange?
About a century ago, a gunsmith came up with the idea of pinching off the end of the barrel. This way, so the thought goes, the edges of the gas cloud will be deflected towards the center. This will keep the cloud from drifting apart for a few yards after it exits the end of the barrel, and so the pellets won't drift apart as fast with the expanding cloud. (Some people insist that it doesn't really work this way. Whatever. The results are the same even if the details are different)
The "choke" increased the range and effectiveness of a shotgun, which means that the situations where a shotgun would be useful also increased. The main problem has been that you needed to buy a completely different barrel for each choke you needed. This has changed in recent years with the development of screw in choke tubes.
I WAS going to list all of the different chokes and discuss which situations they would be suited for, but there's so many that it would be best if you simply asked the guy at the gun store. Remember, as far as self defense is concerned just about any choke will do.