Using Flags on Set

What is the purpose of flags?  How can they be used to shape light? What are cookies?  How do they affect the image? How can you combine these instruments to shape your light?

    The purpose of flags are to eliminate unwanted spill and direct the light. They can be used to shape light by blocking out reflections as negative fill, removing spill that would be on the background or subject, or more dramatically to change the shape of the light, which can alter the focus of the image. Cookies are some kind of form which cuts the light in a particular way, typically in horizontal stripes in order to create shadows that look like window blinds. They affect the image by creating unique shadows that make the light appear as though it is passing through a window with blinds, or some other kind of element that can create shadows. I could combine these instruments to shape light by using a cookie to introduce the pattern to the shadow, then using flags to eliminate any spill and have the light only go where I want it to.