For questions about guitar playing in general. For questions specifically about solid-body electric guitars, see "electric-guitar". For questions specifically about acoustic guitars with or without pickup systems, see "acoustic-guitar".
The 6-string guitar is the modern descendant of a sub-family of the "chordophone" class of vibrating string instruments, from which the bowed string family such as violins, and fixed-length plucked string instruments such as the harp and lyre, are also derived. The name comes from the Latin "cithara" and related Greek "kithara", which themselves are the name of an ancient instrument more like a lyre or harp. The first examples of instruments resembling the modern guitar are from the Andalusian region, about the 12th century, believed to be derived from the European lute and related Moorish oud. These evolved into the Spanish "vihuela" or "viola de mano", a fretted instrument with a narrow, "waisted" body closely resembling the Baroque guitar, which is the direct ancestor of the modern classical and steel-strung acoustic guitars.
Modern guitars come in two basic types; Acoustic guitars descended from the vihuela, which produce their sound primarily through resonance in the body, and electric guitars, which produce their sound by creating an electrical signal that is sent to an amplifier. Both classes can be further subdivided into more specific designs; nylon-strung versus steel-strung acoustic, semi-acoustic and solid-body electric guitars, down to individual body shapes and configurations. Each of these produce subtle variations in the tone of the instrument.
The electric guitar first began to be seen around 1910 in the form of acoustic guitars with various modifications to add a "pickup" element, that sensed the vibrations of the guitar and produced an electrical signal that could be amplified. The first pickups were borrowed from microphone elements and attached to the top of the guitar to sense the vibrations of the whole instrument. This functioned, but produced a relatively weak and artificial-sounding signal. The first pickups designed specifically for guitar were larger magnetic inductors, which sense the actual vibrations of the steel strings due to the disturbances the vibration makes in the magnetic field, inducing a current in the wire coil wrapped around the magnet. These began to be installed on guitars with thicker reinforced tops, and eventually for a number of practical reasons, guitars began to be manufactured from solid planks of wood beginning in the late 1940s. Many classic designs from the 40s, 50s and 60s are still widely available today, having changed very little from their original incarnations.