In the early days strings where made of cat intestines but in reality they mostly where made of sheep intestines.
Nowadays the strings of Spanish guitars are made of nylon.
The first three strings from the six are 'bold' the fourth, fifth and the sixth are winded.
The material that is used for the winding of the strings varies. White or silver coloured rust free steel, nickel, nickel alloy and silvered copper are suitable, but also gold or gold coloured bronze, copper and other alloys.
Most guitarists prefer copper or bronze strings.
In most cases nylon strings are 'roundwound', this in contradiction to 'flatwound' and 'groundwound' strings.
The winding of the nylon core happens mechanical. A long metal thread is used for this procedure.
Rounwound strings provide a reasonable volume and sound.
The volume of new strings is the biggest but while growing older this becomes lesser.
Reason for this event is the fact that dirt, fat and sweat from the fingers affect the metal of the winded strings.
Cheaper strings have the tendency to oxidize after a period of time.
Keeping your strings clean will result in a longer life time and a better quality of sound.
A way to clean your strings is raise the string and let it snap against the fingerboard.
Generally the life time of the strings will depend on the maintenance, the frequency of playing and the use of the register (high or low positions).
There are guitarists that prefer the sound of older strings above the metallic sound of new strings.
The thickness of the strings is given in gauges, mostly inches or better fragments of inches.
Classical nylon strings are obtainable in different gauges and tensions which results in a difference of tone and action. All a question of preference.