Stacks on the desktop keep files neatly organized in groups. Whenever you save a file to the desktop, it’s automatically added to the appropriate stack, helping to keep your desktop clean. You can group stacks by kind (such as images or PDFs), date (such as Date Created or Date Last Opened), or Finder tags. For example, all the documents on your desktop can be grouped in one stack while screenshots are grouped in another.
The Dock comes with a Downloads stack where you can easily find items you download from the Internet, receive as attachments, or accept using AirDrop. When you add a folder to the Dock, you can choose to display it as a stack.