The Brahui language is mainly spoken in Balochistan, Pakistan, although also in Afghanistan and Iran.
According to [1] it has about 2,000,000 speakers in Pakistan (1998), or 2,210,000 in the world. In Pakistan it is mainly spoken in the Kalat region of Balochistan.
Most speakers can not read or write the language. Many are bilingual in the Balochi language.
Although it is a Dravidian language, it has been heavily influenced by other languages spoken in the area and shares few words with the Dravidian languages spoken in southern India and Sri Lanka.
It's sometimes speculated that the use of the Brahui language in Pakistan is a legacy of the Indus Valley Civilisation, and that its separation from the other Dravidian languages indicates they were formerly much more widely spread. This relates to the controversial Aryan invasion theory of ancient Indian history.
References
[1] Ethnologue - http://www.ethnologue.com/show_language.asp?code=BRH
