So many strings, so many gauges. What does it mean, anyway, to use one gauge or another; what difference does it make?

On strings made of one material, like plain gut strings, the only way to change the mass of the string is to make it thicker or thinner, thereby making it correspondingly heavier or lighter for a given length. On strings of fixed pitch and length, thicker strings increase the tension and thinner strings decrease the tension.

Learn more about the difference in string gauges here!