Acoustic guitar strings are made in different gauges and types. New strings from companies like Elixir promise more durability, and strings can be made for specific types of guitars as well.
Classical vs. Steel Strings
Classical guitar strings are made of nylon and produce a very delicate tone. Steel string guitars are what most people think of when they think of acoustic guitars; these strings have better projection and a more aggressive tone. Steel strings cannot be used on classical guitars as they are not strong enough to hold them.
Phosphor Bronze
Phosphor bronze is the most common material used on the windings of the bottom four strings of an acoustic guitar, though bronze itself is sometimes used. Bronze tarnishes faster, and bronze strings need to be replaced more often.
Coating
Elixir started the coated string revolution, and it has since been taken up by other companies, such as Martin and D'Addario. Coated strings have a thin coating of a plastic material that supposedly improves the strings' durability.
Gauge
Acoustic guitar strings are available in sets with a variety of gauges. Medium strings are probably the best compromise between tone, projection and playability, but some players like light strings for more bending ability and others prefer heavy strings to get a woodier tone.
Windings
Rounded windings are the most common on acoustic guitar strings, but some players use flatwound and hexagonal strings as well. On rounded strings, the bronze wire is wrapped tightly around the core string in a round pattern. In flatwound strings, the profile is flatter, while hexagonal strings have a hexagonal shape that prevents the winding from rotating around the core.
