Next to the Strings, the pickups are probably one of the most important factors in sound production and tone on an Electric Guitar.
To the beginner, they're normally shrouded in mystery and left to their own devices. To the professional, however, they play a huge factor in changing the tone and sound of the guitar.
So what actually is a pickup?
In layman's terms, the pickup is the part that literally 'picks up' the sound of the strings. There are lots of shapes and sizes, but they all work by using a magnet with lots of tightly wound copper wire, which creates a magnetic field. As you strum the strings, the magnetic field is disturbed and the guitar pickups convert that disturbance into audible noise through the amplifier.
With me so far?
Good. Now there are many, many types of pickup - too many to cover in one article - but the majority can be split into two categories - Humbuckers or Single Coil pickups.
Single Coil
Probably the most easily recognisable, the single coil is exactly that - one strip across the length of the strings. These are most commonly seen on Stratocaster or Telecaster style guitars, often with 3 separate positions (Neck, Body & Bridge).
Budget - Rocket S300
Intermediate - Yamaha Pacifica 112
These generally deliver a bright, solid 'twang', demonstrated most ably here by Eric Clapton on his signature guitar -
Humbucker
Most commonly seen with a cover, as on the left, they can also be seen without - in which case they just look like 2 single coil pickups (above) placed next to each other. Humbuckers are so called because placing the two single coils together (using magnets of opposite polarity) cancels out any background hum. They also thin out the higher frequencies, giving a guitar tone which is very warm and thick at the lower end. Most often seen on Gibson SG and Les Paul style guitars, they're ideally suited to music which demands a higher level of distortion or gain such as classic rock or metal. What you may see is guitars combining both pickups on the same model - often 1 humbucker at the bridge and 2 single coils at the neck and body, though this isn't a configuration set in stone! Some models, like certain Yamaha Pacificas, also offer 'Coil Tap' - this effectively splits the humbucker into two single coils, giving you access to the different tones - achieved just by pull out a tone pot!
Budget - Rocket L-Zebra
Intermediate - Rocket A-300 Standard Jazz
Advanced - Yamaha Pacifica 120
An idea of this is demonstrated again by Eric Clapton (He played on a mixture of Gibson Les Pauls, SGs and one particular ES-335 with Cream & Blind Faith, before moving to a Strat in 1969 - due to in part Steve Winwood's use of one. He later bought 6 original Fender stratocasters in 1974 and gave one each to Pete Townsend, George Harrison and Steve Winwood. I'm still waiting for mine!)