Baking soda:  A leavener which causes baked goods to rise when combined with an acidic ingredient, such as buttermilk, applesauce, etc.

Baking powder:  Contains both baking soda and an acid which activates it when combined with moisture. 

Brown sugar:  Available in light or dark, which are interchangeable in a recipe.  However, dark has more molasses in it and thus has a stronger flavor.

Stick butter:  Adds flavor, color and texture, and comes in salted or unsalted, which are interchangeable in a recipe, but I prefer to use unsalted. Some bake with stick margarine, but if diet permits, butter is a better. 

Semi-sweet baking chocolate: Adds a rich and sweet flavor, as well as fat to a recipe. Cocoa powder, which is lower in fat, is often used in recipes, as well.

Eggs:  Use large eggs in baking, if diet permits or egg substitutes.  However, eggs add flavor, color and many other important attributes.  

All-purpose unbleached flour:  Gives structure to baked goods. Comes in both bleached and unbleached, which are readily interchangeable. Some add whole wheat flour to their (freezer) pantry, as well.

Milk: Provides moisture, color and flavor.  Use regular (whole) milk or 2% in baking.  Also comes in 1% and skim.  Soy, rice and other milks can be used, as well.

Quick-cooking oats: (Quick Oats) Adds flavor and texture. Use quick-cooking, instant in baking.

Powdered Sugar:  Also called confectioner's sugar, it is finely crushed white sugar.  Do not substitute it with regular granulated sugar.

Sugar:  Known as granulated, white sugar or just "sugar" in recipes. It is the most commonly used sweetener in baking and it performs many important functions. Sugar substitutes can also be used, and go for the granulated kind.

Salt:  How quickly salt dissolves is the key in baking.  Regular table salt dissolves most readily, and is the best choice for baking. But, sea salt can be used, as well.

Vanilla Extract: Adds flavor. Look for the "pure" varieties, not imitation, as pure gives a better flavor.