Acoustic guitar strings are produced in a broad variety of materials, everything from brass to animal gut. But because of the fact that pickups use electromagnetic fields to detect the vibrations of electric guitar strings, they need to be steel. Or at least predominantly steel. Since the appearance of dedicated electric guitars in the 1950s, manufacturers have plated steel strings in a number of metals with a variety of properties.
Stainless strings provide the best conductivity, resulting in a brighter, cleaner output. However, they are rougher to the touch than other string materials. Stainless steel is a very hard material, and though the strings provide good longevity, they can wear frets more quickly. If your guitar has stainless steel frets, it should be OK to use them without worrying about this.
Nickel plated strings use a stainless steel core winding with an electroplated coating of nickel over the top. They produce a bright, warm tone, and the nickel improves the magnetism of the string. The improved magnetic properties of the strings produce a more dramatic effect on the electromagnetic field of the pickup. This creates a better frequency response and greater level of sonic detail.
Pure nickel strings use a nickel wire to wrap the steel core, rather than a thin plating. This means the clear and bright properties of the nickel are much clearer. They're very popular with players of retro '50s and '60s style music, as they were initially popular during that period. Nickel is a softer metal, so it does tend to wear and deteriorate more quickly than the plated equivalent.
The different gauge numbers next to strings refer to the overall thickness of each string. Thicker strings are thought to produce a louder sound with a richer tone. But they make the guitar harder to play, particularly when bending the strings. The gauge number refers only to the high E string, but the whole set will be spaced at the same intervals for each set. The exception here is so-called "hybrid" sets -- they split the set into the bass and treble strings, using a light gauge for the high strings, allowing easy soloing up the neck. However, they use heavy gauge for the bass side to produce fat, crunching power chords.
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