(Italian, "castrated", plural of castrato, derived from Latin castrare, "to castrate")
Male singers, castrated prior to puberty, to prevent their voices from breaking. By this practice, male voices could be attained which equalled or exceeded the beauty of female voices, and had all the qualities of depth, power and endurance contained in male voices. Castration to this end enjoyed its greatest popularity in the 16th through 18th centuries.
The castrati found a natural career in opera, and it was in this field that the most famous castrati were active - these being, among others, Farinelli (Carlo Broschi) and Senesino (Francesco Bernardi). The last known castrato was Alessandro Moreschi, who died in 1922.
Addendum, or why some operas aren't gender-benders, at all:
If you've ever been exposed to a rococo era opera, for example Mozart's La Clemenza di Tito, you may have found yourself wondering what exactly was going on with the casting - or rather, why both voices of the love aria being performed on stage were sopranos, with one of them dressed up in male drag. Was there some subtle sexual innuendo going on, or had you missed something?
The reason for this transsexuality in modern opera is that the male vocal parts currently being sung by sopranos (usually mezzos) were originally written for castrati. Modern casting directors, bereft of castrati, usually choose to rely on female singers for these parts.
I've seen a few modern interpretations, where operas were rewritten for light tenor voices instead. I regret to say that it just didn't work.