The strings are, unlike what people think, very important for example during a performance, bad strings can influence the guitar and especially the sound produced. Not only it will be of lower quality, but also the strings can produce unwanted and unpleasant sounds. I recommand changing your strings every 3 or 4 months. The strings are in nylon and metal, there are even silverplated ones. We also see "composite" strings emerge: their duration is increased.

Here is an overview of the different brands of strings that currently exist:

Adamas

Augustine

D'addario ( my favorites )

D'Angelico

N.Y City Strings

La Bella