Origami has become pretty popular all over the world, so I’m sure you all know how to make folding cranes, or have at least seen them before. Have you seen strings of a thousand before, though? It is relatively common in Japan. Called senbazuru (千羽鶴) you are likely to see them in all kinds of places.
An old Japanese story tells that anyone who folds a thousand cranes will have their wish granted. Why a thousand? The crane is a holy creature that is said to live for a thousand years. So fold a crane for each of those years and he will give you your wish. You can imagine this makes senbazuru fairly popular.
Senbazuru are also a symbol of world peace these days. They became so after WW2 when a young girl who was dying of leukemia as a result of the radiation from the Hiroshima bomb tried to cure herself by folding the thousand cranes. She only folded 600 or so before dying, but her family and friends finished the thousand and buried her with them. Because of this story1, they have become a symbol of peace. This is why you sometimes hear origami cranes referred to as peace cranes.