newars/newa are a linguistic group of nepalmandal (kathmandu valley). it is not an ethnic group but an cultural and linguistic group mixed of different ethnic groups. they have tibeto-burmic and indo-gangetic mixture.
nepalbhasa or newari is basically a tibeto-burmese language with heavy indo-aryan influence. though the grammatical syntax is tibeto-burmese ,the vocabulary is largely sanskrit or prakrit based.
there is an older thread discussing about it in much detail.