A string’s gauge is how thick it is. As a general rule, the thicker a string is the warmer its response will be and the more volume it will produce. However, thicker strings are also stiffer. This makes it harder to fret the string and makes it more difficult to execute heavy string bends. Thinner strings are generally brighter and easier to play, but on some instruments they can sound thin and tinny.
Guitars have been played since the Renaissance era, after descending from the ancient Greek instrument, the kithara. Or, at least, that's how the theory goes: the only real proof for this is the similarities between the Greek word and the Spanish word, quitarra. Early guitars often had four strings, evolving into the six-string version we know today in the late 1600s. Anton Stradivari, the famous violin-maker, also had a hand in making guitars. There's now only one Stradivarius guitar left in existence.