6-string acoustic guitars
— the most popular number of strings in today's guitars are "regular" varieties (acoustic, classical, electro-acoustic — see "Type"). Speaking of such a guitar, they usually mean a six-string instrument. The standard tuning of such models is EADGBE (note that the traditional tuning of bass guitars and some varieties of ukuleles is also based on it).
There are other, much rarer varieties of instruments from this category — a ukulele with a pair of double strings (usually the second and third), as well as a guitalele (the same 6-string guitars, but the size of a ukulele).