Flour is the basic structural ingredient. It's the starch in the flour that will, when mixed with liquid and heated in the oven, gelatinize and set when cooled. Flour also contains the proteins you need to make gluten, which is great for baking bread but not so desirable for making pie crusts. All you have to do is use a flour with a lower protein content, which generally means all purpose flour (cake flour might be too weak).

Fats are considered tenderizing agents for baked goods like cakes and pie crusts, rather than shortening agents like they would be for bread. In pie crusts, fats like oil, vegetable shortening, butter, and lard prevent gluten from getting formed in the first place by coating the flour granules, thus ensuring tenderness. Flakiness is achieved by the way you mix the dough: larger chunks of fat from less mixing make for more flakiness while more mixing and smaller fat chunks make a less flaky crust. They also add a lot of flavor, especially in the case of butter and animal fats. Lard and butter are also generally considered to have a better feel in the mouth, and lard is reputed to create a flakier crust than other fats.

Water and milk are the most common liquids, while buttermilk, eggs, and cream can also be used. Liquids function as a binding agent: they allow all of the ingredients to be evenly dissolved and incorporated. They also hydrate the the starch and protein in the flour and activate whatever leavening is being used. If you're using a liquid besides water, you're also adding fats, sugar, and acidity which is a good thing. The fats add tenderness, the adds to crust color, and the acidity makes the dough more stable and easier to roll out as well as taste better. Make sure to always use a cold liquid to keep the fat nice and cold, so it'll retain its shape/temperature and produce a flaky product.

Eggs are used for hydrating the dough, creating structure, giving color to the dough, and flavor. The whites are 90% water and the rest protein, so that aids in hydration and structure. The yolks are 50% water with the rest being mostly fat, contributing to hydration and tenderness as well as flavor and crust color.

Sugar adds sweetness and contributes the most to crust color. Ths crust turns golden brown because the sugar in the dough caramelizes as it's baked. Also, the texture of the dough can be changed by using sugar ground to different levels of fineness. For instance, powdered sugar makes a dough that is smoother, even if it doesn't taste as good as granulated.