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.
Games
Heart Pieces are featured in 10 Legend of Zelda games. They look different in most of the games, and below is a picture of how the Pieces of Heart look like in each game.
A Link To The Past
Link's Awakening
Ocarina of Time
Majora's Mask
Oracle of Ages
