Wizard wrote
Any chance of a quick introductory statement on what all the string gauges are, and when best to use what?
Guitar string gauges are usually described by their thickness in thousandths of an inch. When guitarists talk about 'nines' or 'tens' etc they are referring to a standard set of guitar strings, with the first string being nine thousandths (0.009") or 10 thousandths (0.010") thick.
The heavier the gauge, the "bigger" and brighter the tone and the more percussive the attack, while the lighter the gauge, the softer the tension and the easier it is to play and especially bend notes. Your "standard" gauges are usually light gauge.
Some example string gauge sets:
Electric Nickel Wound Guitar Strings
Extra Light .009 .012 .015 .024w .032w .042w
Light .010 .013 .016 .026w .036w .046w
Medium .011 .014 .018 .028w .038w .048w
Acoustic Guitar Strings
Extra Light .010 .014 .023w .030w .039w .047w
Light .012 .016 .025w .032w .042w .054w
Medium .013 .017 .026w .035w .045w .056w
Electric Nickel Wound Bass Strings
Extra Light .040 .060 .075 .095
Light .045 .065 .080 .105
Medium .050 .070 .085 .105
You will also find a bit of variation in these gauges across brands and across different types of strings. Different metal compounds used and different ratios between core wire size and wrap wire size create different feels and tones and the manufacturers each have their own idea of what the perfect gauges are for a set (as do players).
Different styles of music will also lean to particular playing styles - a hard rock player will usually tend to light to extra light gauge to facilitate faster playing, bending and techniques like legato, while a trad jazz player will often use heavier strings (usually flatwound .011s to .013s with a wound third string). Also a classical guitarist will use lower tension (sometimes called soft tension) nylon strings for the fuller, more rounded tone, while a flamenco player will use a much higher tension (hard tension) strings for the percussiveness and brightness.
