Money is any asset unconditionally accepted by all in all transactions. At different times in history, different items have been used as money, such as stones, salt, cattle or shells. Metals, gold in particular, have extensively been used. The modern paper form of money dates back to the Middle Ages when it was first used by Venetian merchants.
In some parts of the world, salt is used as money even today. Salt is obviously not an official currency of any nation, but it is the people who accept it in any transaction, who make it money. These people accept salt in exchange for other commodities because they know that they will be able to offer that salt when they want to acquire something else. Salt is convenient because it is easily divisible and because it does not deteriorate.