Guitar strings come in many different thicknesses, but the most common variations fall into two different categories: light gauge strings and heavy gauge strings. Light and heavy gauge strings each have their own pros and cons.
Guitar string gauges are measured by the thickness of each string in inches. Standard electric guitar strings have a thickness of between 0.009 to 0.010 inches for the high E string to 0.042 to 0.046 inches for the low E string. Any set of electric guitar strings thinner than this are light gauge and any set thicker are heavy gauge. Some guitar string companies sell strings with thick lower strings and thin higher strings, but this information is always written on the front of the package. Standard acoustic guitar strings are between 0.011 to 0.012 inches for the high E and 0.049 to 0.054 inches for the low E.
For beginning guitarists, lighter gauge strings will be much easier to play. The thicker the string, the more it will cut into non-calloused finger. However, many guitarists eventually decide that they like the way their guitar feels and plays with thicker strings. Bending strings requires more strength with thicker strings, but the sound tends to be fuller and heavier. Light strings are easier to play leads with and can still give a guitarist a very strong sound. The main thing to worry about is how hard you play the guitar. If you strum very hard, you will probably want heavier strings, and if you have a lighter touch, you will probably like lighter strings.
One of the main reasons that guitarists begin using heavy or light strings is because they want to detune their instrument. Regular gauge strings will sound loose and messy when tuned down, and when tuned up they run the risk of breaking. If you are going to change the tuning of your guitar, you will want to get thicker strings if you are tuning down or lighter strings if you are tuning up.
Most guitar string companies display the words heavy gauge or light gauge strings on the packaging of the strings, but sometimes they use specific names (like slinky) or no designation at all other than the thicknesses of the strings in inches. Somewhere on the packaging of all guitar strings is the specific thickness of the strings. The thinner the strings, the lighter the gauge. The thicker the strings, the heavier the gauge.
Heavy gauge strings add a great deal of tension to a guitar neck. If the truss rod--the rod inside the guitar neck that provides the counter force to the tension of the guitar strings--is not properly set up for heavier strings, you run the risk of damaging your instrument. Owner of United Lutherie Gene Imbody suggests that simple truss rod adjustments can be made at home, as long as you are careful not to force any adjustments. He does, however, warn that use of proper tools is necessary in making these adjustments and if you do not have or know what the proper tools are, you should take your guitar to a repair person.