In classical music and opera, voices are treated just like musical instruments. Composers write music for these instruments, understanding the skills and vocal properties of the singers. Singers build careers by specializing in certain categories on music.
To help both composers and singers, voice classification systems evolved. There are many systems. Some consider how loud a singer can sing. Some consider how fast a singer can sing. Some include non-musical characteristics like what a singer looks like, how well a singer acts, or how funny a singer can be. In Germany, opera houses use a complex sorting procedure, called the fach system, that considers all these elements.
Classifying singers by range and sex is the most common method. But even using these guidelines, there are still many, many categories. The six most common classifications are:
Female voices:

• soprano, the highest female voice
• mezzo-soprano, the middle female voice
• contralto, the lowest female voice, called alto in chorus music

Male voices:

• tenor, the highest male voice
• baritone, the middle male voice
• bass, the lowest male voice

There are many other designations, including soubrette, heldentenor, bass-baritone, coloratura, and basso buffo. There are even categories for men capable of singing in the female range. This type of voice is rare, but still used in opera. In Baroque music, many roles were written for castratos, male singers who were castrated as boys to prevent their voices from changing. Today, with training, a man can still sing these roles. This singer is called a sopranist, countertenor, or male alto.
Singers may also be classified according to the style of music they sing, such as soul singers or carnatic vocalists.