Real Gut Strings
"Cat Gut" is made from the intestines of the sheep or goats. For a long time, catgut was the most common material for the strings of harps, violins, and viols, as well as other stringed musical instruments, although most musical instruments produced today use strings with cores made of other materials, generally steel or synthetic.
There are two types of Nylon strings. Extruded, and Ground. Extruded strings are made by pushing hot nylon through a small hole ground strings are furst extruded and then ground down to size.
Basically fishing line. A brighter sound than nylon string.
A very technical description of Florocarbon can be found on WidipediA:
Synthetic Gut Strings
Aquila has a trade name Nylgut. A synthetic material created ti have to feel and sound of traditional gut strings with out the traditional problem os gut strings.