Expensive, powerful, elastic string with good tension stability that was originally made from sheep's gut but now exclusively (I believe) from cow's gut that's often considered the gold standard for tennis strings. Due to it's relatively high price and susceptibility to moisture damage (despite modern coatings) it's probably not the ideal choice for most recreational or club players.
A term, originally termed by marketers, to describe and add value to inexpensive nylon mono-filament (solid core) strings although in practice many nylon mono-filaments now incorporate other materials and an outer coating or wrap to improve wear or tweak the string's properties. The term systhetic gut is also sometimes used more generally (some say incorrectly) to any synthetic string that tries to emulate the playing characteristics of natural gut.
Very elastic with good tension stability but susceptible to moisture damage even with modern coatings and expensive!
Unfashionable since the introduction of polys but elastic with good tension retention and extremely good value. In my opinion nylon monofialemtn