Cookie sheets come in various shapes and materials. Some are better for darker cookies, while others bake cookies light and crisp. Read recipes carefully—most will tell you which to bake the cookies on for best results.
Nonstick
Nonstick cookie sheets are the most common. They bake cookies more quickly than other types of sheets, but that also makes it easy to overbake and end up with dry cookies. Darker finishes bake cookies darker, and lighter finishes bake cookies lighter.
Cleanup is easy, but after each wash, check the surface to make sure the finish is still intact. Replace when the nonstick finish begins peeling.
Aluminum
Aluminum cookie sheets bake cookies evenly since the metal conducts heat well. Cookie sheets made with thicker aluminum hold heat better than thinner ones, keeping cookies warmer longer once removed from the oven. Highly acidic cookie dough can have a negative reaction with aluminum that gives cookies a metallic flavor; use a silicone baking sheet as a barrier during baking in this case. Aluminum cookie sheets may have rolled edges, high edges or no edges.
Insulated
Insulated cookie sheets have a thin layer of air or a hollow space between two metal layers. These are best reserved for very delicate and lace-style cookies, which tend to bake slowly and require less browning. This type of sheet is great for cut-out sugar cookies since it keeps the cookies lightly colored. If you use insulated sheets for baking any other type of cookie, remove the cookies from the oven as soon as they brown around the edges or they will overbake and become hard.
Black Steel
Black steel cookie sheets are thick and heavy and need to be seasoned before and after every use. Since black steel is a good heat conductor, cookies bake evenly but also very dark—so this type of sheet is good for baking dark and crisp cookies. The pan may require a light oiling so the cookies don't stick.