I’m glad this came up. I bought some “fresh” scallops from a local store a couple years ago and they were horrible. They had a very strong chemical stench and taste. Now I know why.
Zest is the extremely thin yellow layer of the peel. Pith is the white part of the peel. The zest has essential oils and will add a very lemony aroma to a dish, without making it sour. Pith will add bitterness and not much else.
Lemon zesters and microplanes are ideally suited for scraping off that essential part without getting much pith. I wouldn’t want to eat a cake that had whole lemon peel grated into it
And nothin’ beats a microplane for ginger-root! They’re even ok for wood, but don’t try them on the lathe, like one can with regular rasps and abrasives: bad things happen.