A cookie's texture is determined by the ratio of fat to flour, as well as the amount of water present. Generally, a higher ratio of fat to flour will result in a softer, more delicate cookie, while a higher ratio of flour to fat will produce a cookies that are more crisp.
The type of fat used in a recipe also affects a cookie's texture. Butter, for example, contains water and will produce a softer cookie than one made with shortening, which has a higher fat content.
The amount of water present in a cookie dough also affects the final texture. A dough with more water will produce a softer cookie, while a drier dough will yield a crispier cookie.
In summary, the ratio of fat to flour, the type of fat used, and the amount of water present are all factors that will affect a cookie's texture. By experimentation, you can find the combination of ingredients that yields the perfect cookie texture for your taste.