I've always wanted to know

There are certain baking questions that I often ponder but then forget to look up when my hands (and kitchen) are clean enough to do so. I'm not sure you'll be all that interested in this but here it is - mostly for me to read the next time I forget the answers!

Q. What is the difference between white and brown sugar and can I substitute one for the other?
A. Brown sugar used to be made by adding molasses to sugar syrup before crystallization but today it's just white sugar with molasses added after that process. In spite of their difference in weight, you can substitute brown sugar for granulated white on a 1 to 1 basis, and the most significant difference will be taste.
Substitute white sugar for brown sugar on a 1 to 1 basis, but add 1 tablespoon of molasses per cup, and decrease the total amount of liquid in the recipe by 1/2 to 1 tablespoon.
To use honey in place of sugar, use 7/8 cup for every cup of sugar, and reduce the liquid in the recipe by 3 tablespoons.

Q. Why do you use salt in almost every baking recipe?
A. From the research I've done, I think it's all flavor, nothing structural.

Q. Why should eggs be room temperature before baking with them?
A. The idea is that when you beat room temperature eggs, they will have greater volume than their cold counter-parts. This is especially important for whipping egg whites. For cakes and such, the best answer that I can find is that ingredients mix better and emulsify when they're the same temperature and the cake will rise and bake more evenly if everything is the same temperature. If you forget to take your eggs out of the refrigerator ahead of time, you can put them in warm water to bring them to room temperature.

Q. What's the difference between baking powder and baking soda?
A. Both are leavening agents which make baked goods "rise" and baking powser actually contains baking soda. Baking soda is sodium bicarbonate and baking powder contains sodium bicarbonate, but it includes the acidifying agent already (cream of tartar), and also a drying agent (usually starch).
Baking powder is available as single-acting baking powder and as double-acting baking powder. Single-acting powders are activated by moisture, so you must bake recipes which include this product immediately after mixing. Double-acting powders react in two phases and can stand for a while before baking. With double-acting powder, some gas is released at room temperature when the powder is added to dough, but the majority of the gas is released after the temperature of the dough increases in the oven.
You can make your own single acting baking powder by combining 2 parts cream of tartar with 1 part baking soda and 1 part corn starch.

*I found these answers various places and jotted them down to myself without the sources. If you originally wrote anything that I've excerpted above, I give you all the credit and I sincerely apologize.