First, lets define what they are:
Zests are the scrapings of the brightly colored peel, be it yellow, green, or orange. There, we have also handled what Peel is!! Don't go too far, though or you will hit the membrane that is very bitter. To quote "Joy of Cooking" by Rombauer and Becker,
"What better name than "zest" could be found for the gratings of the colorful outer coatings of lemons, oranges, tangerines, and limes - those always available, valuable, yet somehow not fully appreciated ingredients!"Zests brighten up a dish, just a teaspoon of zest can make such a pleasant difference to a dish! Zests, I have found, need a certain grating system to be successful, not just any grater works well. Microplanes are the best, a box grater can work well too. In this video, you can see how they work:
The Peel of the fruit is just that, the outside colorful skin of the fruit. Zests are made by grating the peel...see above.
Juices, directly from the fruit is the best and only true ingredient when a recipe calls for the juice of "such and such" fruit. period.
True juice is not from concentrate with water added, it is the real deal, squeezed from the actual fruit. It is more expensive, but oh so worth it. Just a little goes a long way here too!
So how much is the right amount and what can be a substitute?
"Joy of Cooking" says
1 tsp of freshly grated zest (peel) = 2 Tablespoons of fresh juice = 1 tsp dried zest = 1/2 ts extract = 2 tsp of grated candied peel(zest).
Hope this helps!!