Steel‑string acoustic
For acoustic steel‑string
Material: Strings 1‑2 are steel. Strings 3‑6 have steel or silk cores wound with brass (bright), bronze or phosphor bronze (last longer). My personal preference is phosphor bronze, as the tone is well-balanced, and they last a long time.
Gauge: Light ‑ easier to play and bend, sweeter treble
Medium ‑ louder overall, somewhat richer bass, usually last longer
Heavy ‑ don't buy; they're hard on guitars and void most guitar warranties
For nylon string
Material: Strings 1‑3 are a single, solid, nylon strand. Strings 4‑6 are multi-stranded nylon with a metal alloy winding around the strands. These look like steel strings, but they are not. Don't ever put steel strings on a nylon string guitar - they will literally destroy it!
Gauge: Normal tension and hard tension (also called high tension)
For electric
Material: Strings *1‑3 are a single wire. Strings 4‑6 are a wire core wound with a nickel or stainless steel winding. Nickel has magnetic properties necessary for use with pickups.
Gauge: Extra Light - Terrible intonation, thin sound, break easily . . . but super easy to bend and play. I do not recommend extra light strings for any type of playing.
Light ‑ easy to play, easy to bend, good tone balance with a wound 3rd string
Medium ‑ louder (acoustically), rich bass, last longer, are harder to bend
*Note: Electric sets can be purchased with a wound 3rd string. I hugely prefer wound 3rd strings these because the string stays in tune much better (acoustic guitar 3rd strings are always wound), has a richer tone, eliminates the twangy quality of an unwound 3rd string, and has a better timbral match to adjacent strings.