Guitar strings have undergone significant changes over the past 100 years. After they had been made from animal gut for several hundred years, the availability of nylon caused string manufacturers to rethink how they made classical guitar strings during the early 1900s. In addition, the invention of the electric guitar in the 20th century forced string makers to experiment with strings made of metals like bronze, nickel and steel.
There are three main types of guitar strings: nylon (for classical and folk guitars), bronze (used for acoustic guitars), and steel/nickel (used for electric guitars).
Until the early 1900s, all guitar strings were made from animal gut, and were referred to as "gut strings." Today, strings are made from synthetic materials like nylon and metals like bronze, nickel and steel.
"Plain" strings are made of only one type of material--either nylon or steel--while "wound strings are made by wrapping either steel or bronze around a "plain" string, which thickens the string.
The thinnest guitar string currently in production is .007mm in diameter, while the largest that is commonly available is roughly .06mm.
Famed classical guitarist Andres Segovia is credited as the catalyst in bringing nylon strings to attention of the classical-guitar community.
Using a wound third string on an electric guitar can help produce a "thicker" tone while improving the intonation on that string at the same time.
There is a small number of string makers, mostly in Europe, who still use animal gut to manufacture their classical guitar strings.