The currency in Bali is the Indonesian rupiah. 

You’ll see it written as IDR and it’s the only currency accepted in shops and restaurants. Some larger hotels may accept US dollars and euros, but will generally charge you extra for the privilege.
Notes come in denominations of 1,000, 2,000, 5,000, 10,000, 20,000, 50,000 and 100,000 IDR. Coins are less common, but come in 25, 50, 100, 200, 500 and 1,000 rupiah. All of those zeros can be baffling, but it’ll soon become second nature. Do take particular care when parting with a 10,000 or a 100,000 note, though; they look very similar.
 
Which currency to take?

You’ll get a far better exchange rate in Bali than in the UK, so it’s best to change your British notes on arrival. Bali currency changers accept all major currencies, so make sure you have some new, clean and crisp notes in high denominations, which will speed up the exchange and often get you a better rate. Avoid the money changing stands in the airport as they will be offering a lower rate of exchange than those outside.

Travellers cheques’ can be useful, but try to keep them as a backup, since the rate of exchange will be lower than cash. And remember to take your passport out with you if you’re planning to cash them in. Credit cards are great for larger purchases in hotels, department stores and larger restaurants. But it’s always best to check before running up a bill, as credit cards are not as widely accepted in Bali as we’re used to in the UK.

A good alternative to getting your currency in Bali is a prepaid travel card. You can load it with credit before you arrive and top it up during your stay, so that the money you need is always to hand. You can then use it like a credit card to pay for purchases and withdraw cash from ATMs. You’ll easily find cash points in the southern tourist spots around the capital and Kuta, as well as in the central region surrounding Ubud. They are much rarer in the north and eastern reaches of the island.

Finally, it’s worth pointing out that due to the low value of the currency and the lack of coins in circulation, there’s a tradition of offering nuts or sweets in lieu of lower denominations in shops in Bali. Given the small amounts in question, it’s usually best to accept the sweets and nuts offered with a smile.