What is ..?

 What are contratenors?: Type of classical male singing voice whose vocal range is equivalent to that of the female contralto or mezzo-soprano voice tesitura.

What is a castrati?:  A male singer who, to preserve his high voice,was castrated  before puberty.

What is a beatbox?: An electronic divice adds a backbeat manipulates sounds and a mimics musical  instruments.

What are trebel choirs?: A singer with male and female voice until 16 years.