You can tell whether or not strings are of a thin or thick gauge based on the numbers on the package. The smallest number, which is the gauge of thinnest string, will usually be .9 or lower on thin gauge strings. On thick gauge strings this number will be .12 or higher. Strings that are .10 or .11 are generally considered to be “mediums”, and produce a tone and feel which is the middle ground between these two extremes.
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.