A pickup device acts as a transducer that captures mechanical vibrations, usually from suitably-equipped stringed instruments such as the electric guitar, electric bass guitar or electric violin, and converts them to an electrical signal which can then be amplified, recorded and broadcast.
For the Techies Amongst You: The turns of wire in proximity to each other have an equivalent self-capacitance which, when added to any cable capacitance present, resonates with the inductance of the winding. This resonance can accentuate certain frequencies, giving the pickup a characteristic tonal quality. The more turns of wire in the winding, the higher the output voltage but the lower this resonance frequency. The inductive source impedance inherent in this type of transducer makes it less linear than other forms of pickups, such as piezo-electric or optical. The tonal quality produced by this nonlinearity is, however, subject to taste, and may therefore also be considered by some to be aesthetically superior to that of a more linear transducer.
The external load usually consists of resistance (the volume and tone potentiometer in the guitar, and any resistance to ground at the amplifier input) and capacitance between the hot lead and shield in the guitar cable. The electric cable also has a capacitance which can be a significant portion of the overall system capacitance. This arrangement of passive components forms a resistively-damped second-order low-pass filter. Pickups are usually designed to feed a high input impedance, typically a megohm or more, and a low impedance load will reduce the high-frequency response of the pickup because of the filtering effect of the inductance.