Citrus fruit may be added to the Gin in various ways. Zest, twist, slice or even parts. The peel has essential oils so the first two are the most commonly used. A slice or part  have is a problem. These bring large amounts of citric acid to the Gin and tonic that cause premature destruction of the tonic gas bubble.
To obtain zests and thin peels from citrus we use a zester. To get twists we get the help of a grater. The twist is narrower than the zest but allows a greater length since it can be removed by making spirals.
Common graters allow even to remove very fine twists we call wires. Smaller diameter of these wires contain exclusively the best part of the fruit skin that has more and better aroma.