Falsetto
When people speak or sing, their vocal cords vibrate.  When a man sings falsetto only the edges of his vocal cords vibrate.  These produce harmonics.  It is similar to playing harmonics on a string instrument by lightly touching the string at a certain point so that only part of the string vibrates.  When a man sings falsetto his vocal range is usually one octave higher than his normal singing voice. When people sing in falsetto, their voice can be breathy sometimes.