For the most part, "light" strings for acoustic guitar are considered to range from .012 (high E) to around .053 (low E). "Medium" acoustic guitar strings are generally .013 to around .056. This used to be more of a crap shoot, but most of the manufacturers seem to have reached a loose consensus on these terms at this point.

If you get into electric strings (nickel steel as opposed to so-called "bronze"), the light/medium designations tend to be more variable; the manufacturers haven't reached consensus on that yet. Generally electric players go for a high E ranging from .011 to .008, although jazz or slide players may opt for heavier strings than that.