The Belarusian currency is called "rubel" though Russian variant of the name "ruble" can be (and is) used with relation to Belarusian money.
Currently there are notes ranging from 10 to 100,000 rubles (Br). No coins are used in Belarus for circulation. Only a small amount of commemorative coins is produced.
For the use in metro there are special plastic cards and tokens. Smart cards (plastic cards with microchips) are used for public payphones.
There are not many vending machines in Belarus and if there is one – it would accept paper notes (as do coffee machines at many commercial centers) or tokens will be sold somewhere at a nearby newspaper stand or other small retailer.
There are numerous currency exchange offices in Belarusian cities to change foreign currency to Belarusian rubles and back. Normally there should be no problems with exchanging US dollars, euros and Russian rubles.