The Iron Boots can be equipped to make you extremely heavy and drop in deep volumes of water. They can be found in the Ice Cavern in Ocarina of Time. When equipped, the make you walk very slowly. They are vital in the game as you need them to complete the Water Temple.
The Iron Boots are also found in The Wind Waker. Although you cannot wear them in water, you can use the Iron Boots to avoid being influenced by the wind.
They once again make an appearance in Twilight Princess. They are found in Link's home village of Ordon after defeating the Mayor in a nice, friendly Sumo Match. They are used to ward off the Gorons on your way up Death Mountain, as well as the secret to defeating both the Goron Elder before you enter the Goron Mines. They are also used to defeat Fyrus, the twilit monster that is attached to the Goron patriarch, Darbus. The difference between the Iron Boots in the Twilight Princess as opposed to previous Zelda games is that the Iron Boots are magnetic, and allow link to walk on roofs and walls that possess a certain material used in the Goron mines.