Technically speaking, the zest is the colorful portion of the peel or rind. The peel or rind refers to the entire skin‚Äîboth the colorful outer portion and the bitter white pith that lies right beneath it. The white pith is bitter and unpleasant, while the zest has the bright flavor of the fruit.
But whether a recipe calls for grated lemon (or orange or lime) zest, peel or rind, you won't want to use that unpleasant white pith. It's the colorful zest that adds the bright acidic flavor of the fruit.
And speaking of zest, here's a neat tip. When grating the zest of a citrus fruit, grate the whole thing, even if you only need a small amount. Place the extra grated zest in small airtight pouches made of plastic wrap, and keep them in the freezer. They're terrific to have on hand when you need to brighten a soup, sauce or dressing. See 7 Ways with Lemons for some fresh ways to use lemons in recipes.