Piece of Heart

Pieces of Heart can be found in most Legend of Zelda games. The amount that Link needs to collect in order to get a complete Heart Container is usually four, but in Twilight Princess it was five. In every other game it is four, so this was probably just a way to allow them to add more sidequests and such in Twilight Princess. Each game has a different amount of Heart Pieces that can be found. Some have 20, and some have 52. They are not rare, though some of them take a long time to acquire, such as the 50 Gold Skulltula tokens required to get one of the heart pieces in Ocarina of Time.