It is a piece of the cow’s tail. It comes still on the bone, and is never sold boneless; in fact, there are many bones in it.
To prepare it for the consumer, the tail is skinned, and then the surface fat is trimmed in North America to a 1/4 inch (1/2 cm) — 1/2 inch (1 cm) maximum allowed, as per the oxtail definition in Institutional Meat Purchase Specifications, item no. 721. It is then often cut into 1 to 2 inch pieces (2 1/2 to 5 cm) for sale.
The meat on oxtail is tough, and needs low and slow cooking. But, it contains a lot of fat and cartilage that render during cooking, making it extremely flavourful in soups, stews, and braising.
1 medium sized oxtail will weigh about 3 1/2 pounds (1 1/2 kg), enough for a stew for 4 people.
If buying whole from a butcher, ask for it to be cut into 2 to 3 inch (5 to 7 1/2 cm) pieces.
Oxtail has gone up so much in price, that some people consider it not worth the effort, especially given the hours of cooking fuel that can be required.
Trim away as much of any remaining fat as you can easily do before cooking.
It is not really necessary to soak or precook it, as some recipes call for.
During the first few minutes of cooking, skim off and discard any scum that appears on the surface.
Some suggest cooking it the day before, leaving it overnight in the fridge — this allows you, the next day, to skill all the plentiful fat off very easily, then you can push on with finishing the dish you are making.
You may wish to shred the meat off of it and then proceed with your recipe.
Short ribs of beef