All Suet is fat, but not all fat is Suet. Nor is it always beef fat. The best Suet comes from the fat that grows around the kidneys in cows and sheep. Good cooks are not averse to Suet, even though fat is evil today, because they understand how it works. Butter and shortening melt earlier in the cooking process, blending right into batters. Suet, instead, melts much later, after the batter has begun to set. When it melts, it leaves spaces in the setting batter, making the batter light.
You can buy it in grocery stores in the UK. In North America, it is a bit trickier to procure. If you see any on shelves, chances are it has been at least partially hydrogenated, which creates fats which are worse for you than saturated fats. (For those who you who think, "Suet, pure fat! Must be so unhealthy, I'll use shortening instead" -- think again of all the trans-fatty acids in shortening.)
You can just use one of the substitutes listed below. Or you can make your own. The following assumes that the fat you get will not be the pure fat from around the kidneys, but rather excess fat trimmed off cuts of meat that will contain some membranes, which you won't want in whatever you are making as they will just get stuck in your teeth.
Stick raw fat in a baking dish in a hot over, and melt it. Throw out any solids remaining, and pour carefully into a large bowl to cool. As it starts to solidify, mix in ice water with a hand mixer or whisk until it gets very solid. Pour off the water, and put in fridge to cool and dry. Roll up into balls, wrap in plastic film, and freeze until needed.
That being said, so many British recipes wouldn't call for Suet if they had to go through this all the time. They only call for Suet because it is a bog-standard ingredient that is easy to buy. So use one of the substitutes instead if you're in North America.
You cannot bring real meat Suet in from the UK, but you can bring vegetable Suet in. There are a few different brands of vegetable Suet; all are just fine.
In Canada, you can buy bags of frozen Suet in supermarket chains such as Sobey's and Loblaw's.
If you get balls of Suet destined for bird feeding, check to make sure that they are safe for human consumption, and then purify in the way described above.
If you are going to shred Suet, have it really cold to start with; the colder the better.
Lard, shortening, butter, vegetable Suet, bacon fat.
1 oz / 30g Suet, shredded = 1/4 cup shredded Suet
Freezes well, so don't hesitate to buy more than you need at present, especially if you've had a hard time finding it in North America. And the bonus is that when it's frozen, it grates really easily (many recipes call for you to grate it.)
"Suet was highly recommended for frying purposes. In England nothing but suet is used for frying. It is without odor. It is well to use a small amount of cottonseed oil with it. Cocoanut butter is good but then it is unnecessary to buy any frying material. Mutton suet is preferable to all other suets, but all the fiber must be carefully removed or tallow will be found on the plate on which it is served. In rendering suet keep water away from it." -- Jorgenson, Judith. Around The Evening Lamp. Des Moines, Iowa: Des Moines Daily News. 2 June 1896. Page 2.
Literature & Lore
Comes from an old French word "su" which meant hard animal fat or tallow.