Every baker has a preference when it comes to cookies. Some like them small and crisp, others favor big and chewy. To some extent, these preferences determine your choice of ingredients. Fundamental decisions such as the kind of sugar you use, or whether you make your cookies with butter or shortening, make a big difference to the cookies' final texture and flavor.
Cookies
Cookies come in a wide range of shapes, sizes and textures, but most are based on a relatively small handful of ingredients. These basics include flour, eggs, fat, sugar and flavorings. Depending how these ingredients are combined, the resulting cookies can be crisp, chewy or soft and cakelike. Flavorings and additives like cinnamon, raisins, nuts or chocolate chips make an obvious difference in the cookies' flavor, but changing the fat renders a subtler difference.
The Role of Fat
Creaming the fat and sugar together, to create tiny air pockets, is how cookies become light. Later, during the baking process, baking powder or soda will inflate those tiny air pockets by producing carbon dioxide. Any fat that's solid at room temperature can be used to make cookies, but some are better than others. Lard and beef fat are too heavy for most cookies, and bacon fat too strongly flavored. Edna Stabler's cookbook "Food that Really Schmecks" notes that among her area's old-timers, chicken fat was said to make the crispiest cookies. Shortening, margarine and butter are more common choices.
Shortening vs. Butter
If you're accustomed to making your cookies with shortening, switching to butter will change the outcome slightly because butter is only about 80 percent fat, with the remaining volume made up of water and milk solids. Cookies made with butter will spread more, because of its lower melting point. Shortening makes crisp cookies crisper, while butter gives them a slightly softer crumb. Butter is better for chewy cookies, which require more moisture and less fat. The same holds true for soft, cakey cookies. If you're making large batches, use more butter and less liquid than if you'd used shortening.
Health Considerations
Health-conscious bakers often use shortening or margarine in place of butter, because of butter's high percentage of saturated fats. Unfortunately, shortening and margarine usually contain hydrogenated oils and trans-fats, which can be even more harmful than saturated fats. According to the University of Maryland Medical Center, trans-fats not only increase bad LDL cholesterol, they deplete good HDL cholesterol. As trans-fats are phased out for health reasons, non-hydrogenated shortening and margarine will become more widely available. In the interim, butter is at least as healthy as hydrogenated fat, and better tasting.