String might just be something that you put on your guitar to create sound in general, but the quality and materials of the string can have drastic effects on the tone of the guitar and what sounds it could make.
Guitars
The guitars that are most affected by strings are acoustic guitars, so they will be the main focus of this first article but strings also change electric and bass guitars, just to a lesser extent.
Age
Strings can become out of their element due to how long they have been used, the decline mainly showing through dirt and wearing out.Dirt can get trapped in the wound wire around the metal strings, creating a duller sound, and since said strings are metal they can oxidise, corroding the strings and so tampering with the vibrations leaving a dampened sound (effect of humidity).
The string can get worn out due to fatigue from being used for so long, as constant tuning and playing of the strings will stretch the strings out over time, with more tension being needed to keep that strong and fresh tone, but adding more tension will just stretch out the strings even more, which by then you should probably replace those strings.
Materials
The gauge of the string can affect playing the instrument, like lighter gauges will be easier to play and break, but they produce a lower volume and brighter sound, whereas heavy gauges will be hard to play and tough, with a much darker sound and high volume/sustain.
The materials of the string itself can change the sound, bronze strings have a great feel and make an articulate sound, phosphor strings are warm and rich with a strong bite, brass is bright and almost metallic, while silk and silver have a more gentle touch to it.