What does string gauge actually mean?
If you're new to the guitar scene you may be confused by people saying things like 'I always use nines'. Guitar string gauges are usually described by their thickness in thou' (thousands of an inch). When guitarists talk about 'nines' or 'eights' etc they are referring to a standard set of guitar strings, with the first string being nine thou' or eight thou' thick.

Popular gauges

The best all-round beginner gauges are either 10s (e.g. Ernie Ball Regular Slinky or Rotosound Yellows), or 10s (Super Slinky or Roto Pinks). Even if you want to end up as a Heavy Metal shredder, it's still better so start of with these - if you try to start out with 11s or 12s your fingers will end up so painful that you'll give up! If you want to use heavier gauges its best to work up to them.

It's not that simple, however, as there are many variables in guitar strings not just the thickness of the first string. Most major string brands have a range that allows you to match the top and bottom strings to suit your style... so if you like to easily bend your high strings when playing lead but pound the bottom strings when playing rhythm then you can chose a suitable combination.

As a rule, heavier strings give a fuller guitar sound, but are harder on the fingers for beginners and are more difficult to bend when playing lead. Lighter guitar strings are easier to fret, better for expressive bends, slides and vibrato effects.. but give a slightly thinner sound and break more easily! In my experience, beginners who use 'eights' often break their E string! With Rotosound string sets a spare high E string is included in teh set, but even this won't keep you playing for long with 8s unless your playing style particularly suits them.

It's generally assumed that the first and second string will be 'plain' (just a straight bit of wire), whereas the third to sixth strings will be 'wound' (a thin bit of wire in the middle tightly wound with a wrapping of even thinner wire to make up the total thickness). Where this generality is broken, it is indicated by p for plain or w for wound, added to the gauge. Again, you will hear guitarists say things like 'I prefer a plain G' - an example of this is Ernie Ball Beefy Slinkys, where the 3rd string is '22p'.

Available Gauges

As an example, here's all the Ernie Ball gauges available...
2215 - Skinny Top Heavy Bottom 10, 13, 17, 30, 42, 52
2220 - Power Slinky 11, 14, 18p, 28, 38, 48
2221 - Regular Slinky 10, 13, 17, 26, 36, 46
2222 - Hybrid Slinky 09, 11, 16, 26, 36, 46
2223 - Super Slinky 09, 11, 16, 24w, 32, 42
2225 - Extra Slinky 08, 11, 14, 22w, 30, 38
2626 - Not Even Slinky 12, 16, 24p, 32, 44, 56
2627 - Beefy Slinky 11, 15, 22p, 30, 42, 54

Most of the major manufacturers do similar sets and all manufacturers do at least basic 9s and 10s.