The Structure of the Electric Guitar：What kind of instrument is an electric guitar? – Musical Instrument Guide – Yamaha Corporation

Electric guitars feature devices called pickups embedded in their bodies. Pickups convert the vibrations of the strings into an electric signal, which is then sent to an amplifier over a shielded cable. The amplifier converts the electric signal into sound and plays it. The tone and volume of the sound are also adjusted during this process. In other words, an electric guitar requires an amplifier before it can really be considered to be an instrument for playing music.
Of course, there is no need to plug an electric guitar into an amplifier just to practice, and a hollow body guitar can produce enough sound even when not plugged in. However, an amplifier is required to make the most use of an electric guitar.