Jimi Hendrix might have needed flame-resistant guitar strings, but most of the rest of us are better off looking at characteristics like string gauge, coating and the type of guitar for which the strings are designed. Guitar strings are made to match the type of guitar being played as well as player preferences for sustain, brilliance, longevity and style of play. With all the types of strings available, you need to keep three things in mind when shopping for new strings for your guitar—the type of guitar you play, how light or heavy a string you prefer, and the coating and string material best suited to your playing.
Strings are made specifically for electric, acoustic (steel stringed) or classic (nylon-stringed) guitars, with each having certain characteristics that are not interchangeable. Be sure to choose based on the type of guitar you have.
The size (diameter) of the string is measured as “gauge” and ranges from extra light to heavy. String gauge determines how easy or hard it is to press the string to the fret (and thus hear the note played); it also affects the “bendability” of the string. What works best for you depends on personal preference and the style of playing you do.
Whether a string is coated, and the material with which it is made will affect the string’s brightness and longevity. Coated strings offer some protection from the dirt and oil that can quickly build up on uncoated strings. Some players prefer uncoated strings because they may have a more brilliant sound. The metal alloy used in the string as well as the winding will also affect tone.