A symbol that looks like a capital S with one or two vertical lines through it, the dollar sign represents currency in the US and some other countries and is also used in computer programming.
Oliver Pollack is credited by many sources as being the person responsible for the U.S. $ (dollar) symbol. Seems that his version of the abbreviation for pesos was a little hard to decipher and when the U.S. needed a symbol to represent our money, the $ got the nod. Pollack doesn't always get the credit. Other possible origins include it being derived from the mint mark on Spanish pieces of eight or from a symbol for cinnabar, or from a symbol on a Roman coin. The $ symbol is also used for currency in some countries other than the United States.
One line or two? Usually written with one vertical stroke through it ($), it sometimes is seen with two parallel strokes. Another monetary symbol, the cifrano, uses two lines and looks a lot like the dollar sign. In some fonts the line is written as a short stroke at the top and bottom of the S rather than a solid line through the character as seen in the $ symbol for Courier New (4th from left in the image).
The $ symbol denotes more than money. It's also used in a variety of programming languages to represent string, end of line, special characters, etc. On a standard keyboard, the $ symbol is accessed by typing Shift+4.