RMB Yuan and even sometimes the Yen sign ¥ are all Chinese money.
RMB is the actual currency and it is the short form of Ren Min Bi. Money of the people, or people's money however you want it. This is the defacto currency of China, RMB are proxy US$ btw due to the peg against the US$.
A or a unit of Yuan is the base units for RMB - just as Dollar in USD. This is the currency system in China:
分 Fen cent (pennies for my British readers)
圆 Yuan dollar
So Yuan and RMB is not always the same (as Yuan is just one of the currency units in RMB), but they can often be used interchangeably. 2500 RMB and 2500 Yuan is exactly the same amount of money.