What is a cookie Sheet?

A cookie sheet is a solid sheet on which cookies are baked.  I had always used baking trays until I discovered the ones I use of which I have about four because that way I can have cookies chilling in the fridge or freezer, cookies on cookie trays ready to go into the oven, cookies cooling on the trays before being lifted onto the cooling rack. They are the width of my oven which means I can fit as many in as possible, they are uncomplicated, they are cheap and easy to clean.