The Russian Rouble (рубль) is the legal unit of tender and, freely convertible, and a relatively stable currency. The rouble is divided into 100 kopeks (копейка). Coins circulate in denominations of 1, 5, 10 and 50 kopeks, and 1, 2, 5, and 10 roubles. Banknotes circulate in denominations of 5 (very rare), 10, 50, 100, 500, 1,000 and 5,000 roubles, and there have been discussions about the imminent introduction of a 10,000 rouble banknote although this has yet to be released. The 50 rouble banknote features scenes from St Petersburg, in particular the Peter and Paul Fortress and the Stock Exchange building.
You can purchase roubles in many airports prior to arrival in Russia – while it might be a good idea to have some to hand when you get here, the poor exchange rate offered means you’re probably better off waiting until you arrive. There are ATMs (Банкомат) at the airport where you can extract roubles using most major international bank cards, although be sure to check with your bank about rates and fees before you leave home. Otherwise since recent laws were enacted, currency exchange is now only legal at bank branches (nevertheless you may still come across small exchange bureaux). However there are probably thousands of places throughout the city center where you can change money, so you’ll never be short of options. Look out for electronic signs that will usually display four figures – the current rates for purchase and sale of US Dollars and for Euros respectively. Since the currency turbulence of the late 1990s, all prices are legally required to be displayed in roubles (although you may still see some prices quoted in e.g. US Dollars or Euros). Exchange rates change daily, but as a rough rule of thumb the recent rate to the USD has fluctuated between 27 and 33 roubles, and to the Euro between about 38 and 45 roubles.In larger bank branches you should also be able to exchange other major currencies such as Pound Sterling, Swiss Franc, Japanese Yen, Australian Dollars and so on. Some may refuse to accept dirty or poor quality banknotes (although the mania for entirely uncirculated notes common in the 1990s has thankfully now passed). Avoid changing money in places such as underpasses or if you have to step off the main street and into a back yard to find the office. Common scams include high commission fees not on display, and too few banknotes handed over. Stick to the banks and you should have no problems.
For more information on the coins and banknotes, see the English-language pages of the Bank of Russia (the central bank) website.