For questions specifically about solid-body or semi-acoustic electric guitars. For acoustic and electro-acoustic guitars, or guitar playing in general, see the "guitar" tag.
The electric guitar is a fretted, plucked-string instrument derived from the steel-string acoustic guitar. The earliest examples date from the 1930s and early 40s as attempts to amplify the acoustic guitar without the use of microphones, solving several problems relating to the instrument's use in big-band jazz and blues groups. This class of instrument became very popular in the mid-50s with its incorporation into Western music styles such as country, bluegrass and rockabilly, and its place in U.S. popular music would be solidified with the British Invasion beginning in the late 50s and early 60s.
The first electric guitars appeared in the early part of the 1900s, and borrowed heavily from acoustic designs, but incorporated magnetic single-coil pickups as the primary means of amplification of the string's sound over acoustic resonating chambers. As cost, weight and other practical concerns like feedback from resonating chambers came to dominate construction decisions, luthiers like Les Paul and Leo Fender developed completely solid-body instruments like the eponymous "Les Paul" and the "Broadcaster" (later the "Telecaster") and "Stratocaster".
The typical electric guitar has six strings, tuned to E-A-D-G-B-E in order from lowest to highest, identical to a classical or steel-string acoustic guitar. Alternate tunings abound, however, the most popular being variants of "drop D", where the lowest string is lowered a full step, allowing "power chords" (root-fifth-octave) to be played by a single finger pressed flat across the bottom three strings.
A variety of types and styles of electric guitar are commonly available. The oldest style is the "semi-acoustic", a descendant of the first electric guitars which were basically acoustics with magnetic pickups mounted on the tops. Most modern semi-acoustics are much less resonant to reduce feedback, and incorporate between one and three magnetic pickups, either single-coil or humbucking, plus on-board tone-shaping circuitry. The most popular semi-acoustic style is the "ES" double-cutaway, derived originally from archtop guitars and retaining much of the styling. Full solid-body guitars have no open resonating chambers (though some designs have internal chambers for weight and tone reasons), and include such popular styles as the Telecaster, Stratocaster, Les Paul, and SG; variants of these four basic styles plus the ES comprise the majority of modern electric guitar designs.
Unlike many other instruments used in modern music styles, such as the bass, drum kit and keyboards, guitar players are relative purists, with most preferring very traditional "vintage" guitar and amplifier designs in order to emulate the tones of their role models from older generations. While professional audio in general has evolved from vacuum-tube amplification to solid-state and from analog signal processing to digital, the most highly prized (and most expensive) guitar amplifiers available today employ circuit designs virtually unchanged from the instrument's heyday in the 60s and 70s, and analog, "true bypass" guitar effects units are similarly valued more highly than impedance-buffered and/or digital effects processors.
Despite the general preference, evolution in guitar design has been seen over time, with brands like ESP and Schecter offering low-impedance, high-output "active" pickups and circuitry in their guitars, and digital modelling amplifiers available from most major manufacturers allowing guitarists to emulate multiple types of tube-based amplifier from a single unit. Solid-state power amplifiers combined with more traditional tube-based circuits offer some of the tone of vintage amplifier circuits at a reduced cost and weight. Some materials used in construction have also evolved, mainly due to cost and environmental reasons; thermoplastics such as nylon have been used to simulate animal-based materials such as tortoise and abalone shell, while phenolic resins and non-traditional substitutions have all but replaced wood from the endangered ebony and rosewood trees in fretboards. Polyurethane-based finishes have come to be preferred over lacquers for durability and ease of application (most guitar lacquers are based on nitrocellulose, which is aggressively flammable).
Beginning in the late 1970s, guitarists in the rock genres began seeking more output and less noise from their Stratocaster guitars, which were more ergonomic and easier to play than Gibson's Les Paul but had weaker single-coil pickups. They compromised between the two designs by modifying the Stratocasters to mount Gibson's humbucking pickups in the neck and bridge, creating a hybrid known today as the "Superstrat". Superstrat-type designs became extremely popular in the emerging hard rock, punk and heavy metal genres, and smaller U.S. and international guitar makers like Jackson, Kramer and Ibanez, who were being pressured by Fender and Gibson to stop making copies of their traditional designs, embraced mass-produced versions of this non-copyrighted hybrid to save their companies. These brands became well-known in their own right, and threatened Fender's once-dominant market share with obsolescence. Gibson responded to the metal craze with new, flamboyant body styles like the Flying V and Explorer, while Fender's traditional designs would eventually recover by continuing to appeal to players in the country-western and blues genres, whose influence bled back into pop and rock charts.