The chimney cap is a metal cap or roof that sits on top of the flue liners. Its purpose is to prevent rain from running down the flue liner. It also acts as a spark arrestor so that any sparks coming up your fireplace chimney did not catch your roof on fire. The chimney cap also helps to keep animals out.
Chimney caps are available in a wide variety of sizes and shapes. Tamika apps can range from 50 dollars at local big box store to several hundred dollars for custom fit brass and stainless steel chimney caps.
Often times people refer to the concrete on top of the chimney as the cap. And more accurate time for this is the Chimney crown