Iranian currency is called Rial (RI). If you come across the word Toman in Iran ,do not be surprised. In fact everyone use the term of Toman instead of Rial except in some spacial cases. 10 Rial = 1 Toman.
About rate of exchange there is little fluctuation and credit cards are not accepted in Iran. We recommend you to carry some cash with you while travelling in Iran since official services are not available for credit cards .
It is safe to carry cash with you but it doesn't mean that you don't have to be careful with your cash money.