The mighty Cod has been a "Stable of the Table" for many many years. It is still a fish that is caught in the wild and although stocks vary it is generally a sustainable fish in UK waters. Its popularity has forced the price of the product up in recent years so people do seek alternatives, but none the less it is an excellent eating fish.
The Cod is a "Demersal" fish which means it feeds towards the bottom of the water column. It is a predatory fish living on other fish, small crabs and crustaceans, it can grow to very large sizes making it a target for sport anglers as well as commercial fishing boats. It is traditionally "Trawled" with wide mouthed nets running on the seabed but more recently there has been a trend towards "Line Caught Cod". This is not, as you might imagine, a jolly fisherman with a rod and worm on a hook, it is a boat feeding out dozens of lines with thousands of hooks, the advantage is that this form of fishing does not scrape the seabed, it selectively targets a fish species and tends to get the best fish with the least stress.
Cod are caught in the Northern hemisphere in both the Atlantic and Pacific, the fish are slightly different. In the North Atlantic they are fished from the East Coast of the USA, Greenland, Iceland, Faroes, Shetlands, Northern part of the North Sea and up into Scandinavia.
There are a lot of fish that are closely related to the Cod that are equally good eating, including Pollack, Ling, Whiting and Skrei (Northern Norway)
Black Cod (Sablefish) is not really a Cod but hey, you know the Americans; They don't know the difference between a Boot and a Trunk..