A dollar is a type of currency. Many countries have named their money the dollar, so it is important to say which dollar is being talked about. The symbol for the dollar is a capital letter S, pierced by one or two vertical lines: $.
The dollar is named after the thaler. The thaler was a large silver coin first made in the year 1518. The thaler named after the Joachimsthal (Joachim's valley) mine in Bohemia (Thal means valley in German). The later Spanish Peso was the same size and was often called "Spanish dollar". In the 18th century it became a world currency. Many national currencies were originally Spanish dollars including the ones now called dollar or peso and the Japanese yen, Indian rupee and Chinese Renminbi.