Russian ruble drop in value due to low oil prices
The Rouble depreciates against the US dollar because of demand and supply. 

Think of it as a story that goes as follows: 

a) In the ruble/dollar market, the supply of dollars consists of foreigners that have dollars and want to buy goods(oil and other things) and assets from Russia, while the demand for dollars consists of Russians that want to buy goods and assets from foreigners and need dollars to do so. 

b) When the price of oil collapses 50%, then the foreigners only need to supply 50% as many dollars to buy all the oil they used to buy from Russia. So supply of dollars to the ruble/dollar market shrinks. When supply shrinks, prices increase.  Imagine that as the Russians want to buy foreign goods, say medicine, and they will be willing to pay lots of Roubles to buy said medicine, so demand for dollars does not change as much as supply of dollars.