Although Andrés points out that we have discussed much of this before, I'll add a bit to this thread. Cat at covenhouse, "alto" is the same as "contralto". It is just the shortened version of the word contralto. Contralto means "against high", or the woman's part that is written to sing against the high soprano range part. If you can sing well in the tenor range, then you are an alto (or contralto, the terms mean the same). Andrés provides some excellent information.
Here is a website on singing that goes into detail on the history of castrati, etc. click here The type is light on dark in an impossibly small font size to read, but a bit about the castrati I will quote here:
"Women were not allowed to speak or sing in church. The Church also forbade women to participate in the theater. In the Middle Ages, the lack of female voices in the relatively simple church music was not a problem. Young boys' voices had difficulty, though, with the complex polyphony that was being written in the late 1500s by the contrapuntalists in the Netherlands. Either their voices were not strong enough to maintain the part, or by the time they had gained the musicianship required to execute the music, their voices were changing. Initially this problem was solved by importing falsettists from Spain and for a time Spanish falsettists held a monopoly in the Sistine Chapel. Somehow they seemed to have discovered a secret for giving the falsetto voice more agility, range and a richer sound. Some have suggested that these falsettists were in fact castrati and some may have been. It is documented that in 1599, Pietro Paolo Folignato and Girolamo Rossini (No relation to the other Rossini that we know.), two Italian castrati were admitted to the Sistine Chapel. This, along with the invention of opera at about that time, ushered in the age of the castrati. Castration had existed for centuries as a form of punishment. In other situations, slaves were castrated and then used as harem guards or as servants or tutors for upper class women. Essentially there are two types of castration: removal of all the genitalia (usually inflicted as punishment and often fatal) and removal of the testes only. The latter of these was what was performed on prepubescent boys usually between the ages of seven and twelve. Unfortunately many boys were castrated with the belief that castration alone would make them good singers. It is estimated that at the height of the castrati's popularity during the eighteenth century as many as four thousand boys a year were castrated in Italy. Sadly very few of them became rich or famous.
Henry Pleasants, in his book, The Great Singers, describes the physical results of castration: "The vocal consequences of castration went well beyond the mere perpetuation of a boyish treble. The child continued to grow, and so did his voice; or at least his physical powers to exploit the voice he already had. Under the rigid discipline to which he would now be exposed, his lung capacity and diaphragmatic support would be augmented to an extraordinary degree, enabling him to sustain the emission of breath in the projection of tone up to a minute or more, which is beyond the ability of most normal adult male and female singers. The mature castrato was a boy soprano or alto with all the physical resources of a grown man . . ."
The castrati continued in the Sistine Chapel and the churches long after they fell out of favor on the opera stage. Giovanni-Battista Velluti was the last of the great operatic castrati. Meyerbeer wrote Il Crociato in Egitto (1824) especially for him. Domenico Mustafà was director of papal music until as late as 1902. Unbelievably, there exists a recording of the last castrato to direct the Sistine Chapel Choir, Alessandro Moreschi (1858-1922). Though Moreschi is past his prime, the recording gives the listener an idea of the mysterious beauty of the castrato voice."
A sound sample is provided.
Physically, you have to realize that men develop muscular strength that they can use to support the voice that women don't develop. When the voice is stopped from changing at puberty, (you know how teenage boys develop that adam's apple) then their bodies continue to grow with muscular strength that a woman does not develop You can see that the smaller, high sounding vocal cords would have alot more strength to support the sound, as in the above description of the ability to support the high voice longer than most female sopranos. I can say, that I know sopranos with the right training can sustain a note just as long by knowing how to control the breath. It takes muscle, training, and in-born physical talent.
Women were not allowed to perform on stage in the first operas. The female roles were sung by castrati dressed as women.