If you have a coil of insulated wire wrapped around a magnet you create a simple generator. The pickup generates an AC signal and sends it to the amplifier, via the various pots and switches. The 42 or 43-gauge wire must be handled very carefully, as it's very thin and fragile.
Different pickups utilize more or less turns of copper wire.
This is one way that manufacturers can change the output and tonality of a pickup design. Coils generally have anywhere from 6,000 to 8,500 turns of wire. Depending upon the design of the pickup, magnets come in a wide variety of shapes and sizes. Magnets can also be made from various permanent-magnet materials, such as AlNiCo and ceramic. A humbucker must have two coils that are wired to that they effectively cancel 60-cycle hum.