The official currency of Macau is the Macanese Pataca (MOP). The pataca is further subdivided into 100 avos, with coins denominated in 10, 20, and 50 avos and notes issued in denominations of 10, 20, 50, 100, 500, and 1000 MOP.
However, the Hong Kong Dollar (HKD) is also widely accepted in Macau at a 1:1 rate with the Macanese Pataca, as the two currencies are virtually pegged. If you pay in HKD, you may receive change in either HKD or MOP. This is particularly common in tourist areas and larger businesses such as hotels, restaurants, and casinos. For smaller businesses, especially outside the tourist zones, it is better to use MOP.
Credit cards are also widely accepted in hotels, restaurants, and stores. ATMs are readily available throughout the city for cash withdrawals.
Please note that while MOP is widely accepted in Macau, it is not readily exchanged in many countries outside of Macau, so it's a good idea to use up your MOP or exchange it back into your home currency before you leave.