Nylon strings
Nylon strings were developed to replace strings made from animal intestines used on historical guitars. They are a low voltage string and should only be used on guitars intended for nylon strings. Nylon strings are almost always found on acoustic guitars.
Steel strings
Steel strings are used on many acoustic guitars and all electric guitars with magnetic pickups. Steel strings are higher tension and require a steel reinforcing bar in the guitar neck.
Unwrapped nylon and steel strings
Strings for higher positions on both nylon string and steel string guitars are bare. They do not have a second wire of metal that sows over them for thickness.
Wound nylon strings
The lowest three strings in pitch on a nylon string guitar are wound with brass or bronze.
Wrapped steel strings
On all steel string guitars, the three strands with the lowest pitch have a second wire wound around a steel core to improve how the strings vibrate when the lower notes are played. On acoustic guitars, there are four loops instead of three. Different types of guitars require different types of guitar strings. Some guitars can use one of several types of strings. The type of strings you choose depends on your guitar and the type of music you play.
Source:danspela.com