A cookie sheet is a fairly large flat piece of metal used, as the name suggests, for baking cookies and other foods. (These days you can also buy flexible non-stick sheets of some amazing synthetic substance that does not burn.) Cookie sheets come in many sizes, but The Joy of Cooking recommends 17 X 14 inches as perfect for baking 12 to 16 medium sized cookies.
Cookie sheets can be rimmed or rimless. The rimmed ones have a very shallow lip around the sides, and are useful for containing foods that might spill over the edge (the dreaded spreading cookie-mass) and for placing under overflowing bowls, dishes, and cutting boards to catch the drips. You can also use rimmed cookie sheets as jelly roll pans. Rimless ones are useful for sliding cooked food off of with style and panache.
Cookie sheets can be shiny or dark coloured; the latter will cause food to brown more quickly. They can be non-stick, in which case you must take care not to scratch them with spatulas or scrubbers. They may be insulated, in which case they will have two layers of metal separated by a pocket of air. These are not really worth the extra expense, in my opinion.