A stack is any program window. It just designates the program window frame and its decorations. Any stack can have multiple cards.
A card is where you place the controls, so you can change the inside of a windows appearance just by changing the card of the stack. Just to give you an example, it is similar to the installation programs where you press the next button.
Moreover a program may contain many windows, any other windows of your program are called a substack. Substacks are like stacks, the can have multiple cards.
This is another example: