Exchange
For exchanging money, you can refer to the banks which have a sign of "EXCHANGE" on their entrance.  There are also authorized exchange shops in the airport or tourist areas where you can exchange your money at a bit more cost, but fast.  These shops are called "SARRAFI" in Farsi which is the equivalent word for exchange.
There are also street exchangers in tourist areas who usually hold a bunch of banknotes and show them to people who are passing and shout Dollars, Pounds, Euros. This way of exchange is not legal and is not recommended.
You can exchange USD, EUR, GBP, AED almost everywhere, but other major currencies can be exchanged in certain bank branches.
Foreign credit cards are not still a reliable way for payments in Iran, as the situation of their acceptance changes a lot. Sometimes, you can use it and sometimes you can't. So, it's better to forget it at least at this time. Hopefully, according to the new financial laws, this problem will be solved soon. There are many ATMs everywhere in Iran, but they only accept the domestic cards and are currently useless to foreigners.
Travellers Cheques may be accepted by some hotels or shops, but it's not a reliable way of bringing money to Iran. However Bank Melli branch at Tehran International Airport usually exchanges major international Traveller cheques.
Considering the above matters, it is recommendable to bring money in cash in form of USD, EUR or GBP. Using a money-belt is proposed.

Current rate of exchange according to Iran's Central Bank.
This is usually a bit lower than the street rate.

Iranian Currency

Iranian currency is Iranian Rial. There is another money unit in Iran which is called Toman and is equivalent to 10 Rials. Toman is more common in oral quotes, while Rial is the official unit and used in most verbal documents. Price etiquettes may come either in  Rial or Toman and usually are understood by the buyers. In case of doubt, check it with the seller.

Current Coins & Banknotes
 

Regulations on Money Export/Import