Pie crusts are usually made with butter, shortening or a combination of the two. The solid fats are rubbed into a flour mixture, creating a dough that resembles coarse, wet sand before some liquid is added to it and you can press it together into a ball. The procedure is the same, but the two fats will give you different results in your recipes.
Butter is about 80% fat and 20% water, give or take a few percent depending on the brand and whether you are using European-style butter. In a pie crust, the butter is distributed in small pieces throughout the dough and because the dough is chilled before it is baked, these pieces are solid. When the pie crust bakes, two things happen: the butterfat melts and the water evaporates. The melting butterfat makes the crust tender and a little bit crumbly, contributing to a melt-in-your-mouth type feel. The evaporating water creates a little pocket in the dough where steam tried to escape, giving the crust a flaky, layered texture. The flakiness of a well-made butter crust would have a little bit of the flakiness of a croissant, rather than just the crumbliness of a shortbread cookie. It also has a great butter flavor.
Shortening (and this applies to hydrogenated and nonhydrogenated, although I recommend you work with trans-fat free shortening) is 100% fat. This means that it has plenty of fat to melt into the dough and create a tender, melt-in-your-mouth type of crust, but no water to create a flakiness. A crust made with just shortening will seem a little lighter than a crust made with butter alone, but since shortening doesn’t have much flavor, it will be a little on the bland side. Lard is also a viable option for a fat in pie crusts. Lard – also 100% fat – tends to have more flavor than shortening, but it acts just the same way, making the crust tender and short, but not adding the flakiness of butter.
My personal preference is to use mostly butter and a small amount of trans-fat free shortening in a crust to get the flavor and flakiness of butter, but a little bit of the extra tenderness of shortening. I usually use 3/4 butter and 1/4 shortening, although sometimes I will use a little less shortening. Preference aside, I make all butter crusts like the one pictured above more often than not, since even though they don’t have that little bit of extra tenderness, they are still just as flaky, tender and tasty as I could want.