Stackz is a flascard organizer. It helps you to keep the overview over the words you are learning, even if there are several thousands of them.
To achieve this goal, the words are classified according to two fundamental aspects: known/unknown - known words are on the right, unknownones on the left.
fresh/old - the color of the card indicates the timewithout a positive test. The known/unknown separation is a well accepted technique, it has been widely used in the traditional flashcard approach where paper flashcards are moved up and down the promotion levels.
The Stackz system goes one step further by also showing which cards have not been looked at for a long time. This basic scheme ist just one out of many schemes (Color Modes).