Here are the four main voice types used in operas, from highest to lowest (there are many, many more), but for our purposes, these are a good place to start.
This voice type often plays the romantic female lead or younger female characters.
In this excerpt, this high voice is used for a supernaturally powerful (evil) queen. Wolfgang Amadeus Mozart (1756-91), Die Zauberflöte, K.620 (1791), “Der Hölle Rache kocht in meinem Herzen”, performed by Diana Damrau
Alto or mezzo soprano
This voice type, because it is lower than the soprano, is often used for older femal characters.
Here, the title character of the opera, a defiant and sultry woman, is written for a mezzo soprano. Georges Bizet (1838-75), Carmen (1875), “L’amour est un oiseau rebelle”, performed by Elina Garanca
This voice type often plays the hero or romantic lead.
Giuseppe Verdi (1813-1901), Rigoletto (1851), “La donna è mobile”, performed by Luciano Pavarotti
Bass voices are often used for older male characters, wise men, or comedic relief (a buffoon who is often the butt of jokes).
Jerome Kern and Oscar Hammerstein II, Show Boat (1927), “Ol’ Man River”, performed by Paul Robeson
Another voice: castrati
You’ve read about castrati already, a voice type that was quite admired in the 17th and 18th centuries. It was considered an appropriate vocal timbre for heroic and romantic characters because it was powerful and supple at the same time.
One of the last living castrato singers, Alessandro Moreschi (1858-1922), who had sung in the choir at the Vatican his entire life, made this recording when he was no longer an excellent singer (due to his age), but they give a sense of what this vocal timbre sounded like.