The Nepali currency is the rupee
The US dollar is the currency in which ‘tourist activities’ are quoted and charged, but don’t expect dollars to be useful anywhere except the 2 tourist centres (Kathmandu and Pokhara). See here for the cash/dollars you will need on arrival. ATMs are now widespread in Kathmandu and Pokhara and a range of currencies can be exchanged, but the most efficient means of exchange is with US$ 100 bills at the money exchangers in Thamel (the tourist quarter in Kathmandu). If a sufficient amount is to be exchanged then rates can be negotiated, ask round. NB US$ always have to be in crisp, uninked condition.
Don’t forget to carry a considerable amount of small rupee notes when in the countryside for any purchases you may wish to make, small tips for photos etc.