If you want thin, candy-like cookies, add more sugar. Sugar becomes fluid in the oven and helps cookies spread.
If you want an open, coarse crumb and craggy top, add baking soda. Baking soda reacts quickly with acidic ingredients (such as brown sugar) to create lots of gas bubbles.
If you want a fine, tight crumb and smooth top, add baking powder. Baking powder works slowly and allows for an even rise.
From Cooks Illustrated