Guitar strings can be made of a variety of materials. Acoustic guitars can use both nylon strings and steel strings, though these strings are designed for specific types of guitars (classical guitars for nylon string, for example). Electric guitars also use steel strings, but there are differences between steel strings for electric and acoustic guitars.
The Wound Third
The third string of the guitar, the G string, is a wound string on an acoustic guitar. On an electric guitar, the third string is a plain steel string.
On electric guitar, ferrous metals, such as nickel, are used for the windings on the bottom three strings. This is to help the pickups sense the vibration of the string magnetically.
Acoustic guitar strings are usually wound with non-ferrous materials, such as bronze or phosphor bronze, to give better tone when played without amplification.
On an electric guitar, players often use thinner gauge strings to help with leads and bends. On an acoustic guitar, the strings are thicker to help with projection.
The windings on the upper strings on an acoustic guitar are almost always roundwound, giving more brightness and better tone. On an electric guitar, many players, especially jazz musicians, prefer flatwound strings for ease of playability. With an electric guitar, the loss of tone from flatwound strings is not as pronounced.