Flour-- Read Joy of Cooking (cookbook) chapter on Know your ingredients About flour.
All purpose flour is just that, generally good for most things.
Soft (or Cake flour) is flour that has been milled from a "soft" variety of wheat
Bread Flour is milled from "hard" wheat.
Gluten, a protein of wheat is what defines hard and soft. For Bread, you want gluten, it forms the elastic structure that the yeast works against, and allows the bread to rise. It also give the bread is nice chewy texture.
For cake, you don't want nice chewy texture, you want tender crumbs, for the cake to just hold together. Two factors at work, low gluten and low agitation (you don't "knead" a cake or even mix it for 10 or 15 minutes, as you do with bread)
Winter wheat (wheat sown in the fall, that winters over, and is harvest early in the spring) tend to be a hard wheat. Wondra flour, which is sold in a canister, and marketed as being perfect for thickening gravy and sause, is the the hardest wheat. if you just take some samples, and feel the flour, (cake, all purpose, bread and Wondra) you can actually feel that some flour is softer. Face powder used to be made from (and still often is) wheat flour. if you put some cake flour in you hands, and feel it, and then some wondra, you would quickly realize, you'd want cake flour for your face! you can feel the softness.
And as for buttermilk, you can in most american stores buy small cans of dried powdered buttermilk. It need to be refrigerated after opening, (just like powdered whole milk), but it stays usable for 6 month to a year.
and since this is a word site, and not a cooking class, here is an interesting tidbit, the word Knead is related to the word Lady-- but i don't remember all the detail of the relationship. (this little fact was also in Joy of Cooking!) perhaps some one with a reference library at hand can assist.
