At its simplest, a guitar is "a stringed instrument of the lute family, plucked or strummed, as the instrument sits upright in one's lap." However, the guitar family has proved particularly adaptable to different cultures, musical genres, and to technological developments.
The modern Classical Guitar has six strings, usually of nylon (once of gut), a wooden resonating chamber with incurved sidewalls and a flat back, and is nowadays normally tuned to standard tuning (EADGBE).
The Flamenco Guitar is very similar, but of lighter construction.
The Acoustic Guitar is similar to the modern classical guitar; however, it has steel strings, which involves building a stronger and larger resonating chamber, and a stronger but narrower neck. Standard tuning (EADGBE) is the commonest tuning. However, a wide variety of alternative tunings is available, some of them particularly associated with certain musical styles. The acoustic guitar has proved particularly flexible and some of the variations include
Resonator Guitars - where the sound box's function is replaced by metal resonators;
Guitars with varying numbers of strings; 6 courses of 2 strings (12 strings) is relatively common, but there are also 7, 8, and 10 string guitars;
Guitars with different shaped bodies - with flat tops or arched tops.
Semi-acoustic guitars are designed to combine the physical construction of a sound-box to transmit and amplify sound, with inbuilt electronics to facilitate use as an electric guitar.
The Electric Guitar generally has a solid body, as electronic amplification replaces the function of the acoustic guitar's sound-box. It generally (but not necessarily) has 6 strings, tuned to standard tuning (EADGBE).
Bass Guitars are a particularly flexible development of the electric guitar; they generally have 4 strings.
There are two major caveats:
These definitions reflect a somewhat simplistic view of the commonest guitars as played by most players in major genres of western musics. Beyond that, the guitar family includes
Adaptations of the classic model in Iberia, Latin America, the Caribbean, and elsewhere; these vary considerably in size, number of strings, tunings, and method of playing;
Adaptations which considerably change the structure of the guitar - Steel Guitars (Lap Steel or Pedal Steel), which bear some resemblance to zithers.
The physical description of the "guitar box, strings, and neck" has little connection to musical styles/genres in which the guitar has become irreplaceable. It would be very hard to imagine some musical styles without the guitar, played in particular styles, with a particular range of chords/fingerings, or with specific tunings. These genres require very specific skills and understanding of the instrument. A few examples: