A countertenor is a male singer who uses a falsetto voice to sing high notes like a female contralto or mezzo-soprano.
Countertenors were used a lot in the Renaissance and Baroque periods when females were not allowed to sing in church choirs. They did not have big, dramatic voices like castrati, so they were not normally used in opera. People forgot about this way of singing in the 18th and 19th centuries, although male altos continued to sing in English cathedral and church choirs. In the 20th century an English countertenor called Alfred Deller became very famous. He sang many songs from the Baroque period that had been written for the countertenor voice, for example songs by Henry Purcell who sang countertenor himself. Today there is a lot of interest again in using this type of voice for performing early music like Renaissance and Baroque music.