Zest is the outer-most rind of any citrus fruit. It’s where all the color is and, not coincidentally, all the flavor too. The zest is loaded with essential oil and is crucial to any good citrus-based recipe.
I prefer to use the finest holes of a grater for this task. It takes the zest off in small enough pieces that they can be added right into a recipe. There is a groovy tool called a microplane that does an excellent job of this. (The microplane is a carpenter’s rasp fitted with a stylish handle, and sold for mega-bucks at kitchen gadget emporiums. Any old rasp from the hardware store will do the job.)
A tool I do not like nearly as much is the zester. One might think that because it is named zester, it is the perfect tool for the job. Not so! It strips zest off in long pieces which must then be chopped fine with a knife. (An extra step? No thanks!) Also, the zester frequently digs too deep, pulling up the white bitter pith underneath the zest. Not good.
Take care when grating zest that you do not grate too much off. The fruit should look naked when you’re done, but still round.
Zest can be harvested and frozen for later use. Keep it in a zipper bag in your freezer for last-minute citrus inspiration.