Some musical terms: voices

Let's start with the voices, from high to low:

Female: soprano, mezzo soprano (or mezzo), contralto.

Male: countertenor, tenor, baritone, bass-baritone, bass.

Here are some examples:

Soprano Lucia Popp sings Die Holle Rache from Mozart's Die Zauberflote/ The Magic Flute (1791)

Mezzo-soprano Olga Borodina sings Mon coeur s'ouvre a ta voix from Saint-Saens's Samson and Delila (1877)

Contralto Ewa Podles sings When I am laid in earth (Dido's lament) from Purcell's Dido and Aeneas (1688)

Countertenor Andreas Scholl sings Ombra mai fu from Handel's Xerxes (or Serse): also known as Handel's Largo (1738)

The countertenor voice is not a natural male voice: it is artificial. The singer has to go beyond his natural voice where his voice may break in order to reach this higher sound. If you can imagine a yodel, the higher notes are usually sung in falsetto. You can clearly hear the difference between the natural and falsetto voice in this rather marvellous clip of Franzl Lang yodelling:

In most operas of the Baroque period (roughly mid 17th-early 18th century) the hero was sung by a castrato: a boy castrated before puberty to keep his voice unnaturally high. The high voice would, though, have a certain masculine roughness to it but was also believed to be very pure. The castratos ruled opera for many decades and had many admirers for their combination of vocal purity and virility: the most celebrated was Farinelli. Nowadays, we tend not to castrate boys for them to sing opera, so the castrato roles are sung either by countertenors (though being a falsetto voice, they often lack the power to project in larger opera houses) or by mezzos/contraltos (who have to dress as men which can stretch credibility). The last known castrato was Alessandro Moreschi (1858-1922). Here he is in 1902 singing the Bach/Gounod Ave Maria. (This is not an easy listen. Whatever voice Moreschi may have had, he is clearly past his prime when recording this.)

Tenor Neil Shicoff sings Kuda, kuda vy udalilis (Lensky's aria) from Tchaikovsky's Eugene Onegin (1879)

Baritone Leo Nucci sings Eri tu from Verdi's Un Ballo in Maschera /A masked ball (1858)

Bass-baritone Donald McIntyre sings Leb wohl, du kuhnes herrlishes Kind / Wotan's Farewell from Wagner's Die Walkure (1870) 

The majority of bass-baritone roles can be found in the German repertory, Wagner especially. Although basses and bass-baritones will also sing some baritone roles if they have the high notes, baritone roles will usually hover in the higher notes for longer periods of time: this could strain a bass or bass-baritone voice. It's also a question of colour, bass voices tending to be darker and blacker than baritone voices. Roles such as Don Giovanni, Count Almaviva (in Mozart's Marriage of Figaro), Escamillo (in Bizet's Carmen) and Scarpia (in Puccini's Tosca) are often sung by either baritones, bass-baritones or basses.

Bass Boris Christoff sings the Death scene from Mussorgsky's Boris Godunov (1869)

In the next series of musical terms, we shall look at some common musical forms and structures used to create an opera.