Tibet's currency is also Renminbi (RMB) just like the rest of China. You can also use US dollars in Tibet, but not all the shops accept dollars, and even when they do you don't get a fair exchange rate. Only limited hotels accept credit cards. ATMs are not available everywhere. It is best if you exchange your cash for Chinese Yuan. You can exchange foreign currency in The Bank of China which also has many branches in Lhasa. Only the central office (LingkhorBei Lu) offers cash advances on credit cards like MasterCard, Visa, Amex and Diners Club whereas every other branch cashes out travel's check.
Also, hotels like Lhasa Hotel and Tibet Hotel provide the facility of foreign exchange. As Tibet lacks conversion outlets, you have to change your extra RMB on the black market before leaving.