I'm known as something of a pickup fanatic. Well, maybe that is an understatement. I've tried virtually every pickup out there at some time or another. I've had more pickups than many guitarists have had sets of strings. My long suffering primary guitar alone has had 14 different pickups in the last 10 years. Read on, and I'll try to explain my addiction.

Maybe you have a guitar which is your dream axe in every respect apart from sound. Or you need more power... or bass... or treble...   I find it amazing how many big name guitars (no names mentioned) have mediocre pickups in the vast majority of their models. A good pickup can make a cheaper guitar sound good and a good guitar sound excellent. Less hum, more tone, singing harmonics, at a fraction of the cost of replacing the instrument. And when you find an excellent guitar and put in good pickups...

There are two basic types of electric guitar pickups: single-coil and humbucker. They are both magnetic pickups, using  magnets surrounded by a coil of wire. These are the most common pickups and are responsible for the well known electric guitar 'sound'. When the guitar string moves through the magnetic field of the magnet, the variations in the field cause the coil to generate a small amount of AC electricity. This electricty is then amplified by your guitar amplifier. Variations in magnet strength, shape and composition as well as number of wire windings, wire gauge and composition make for a variety of tones and power output strength.

Single-coil Pickups (eg. Fender Standard)
The simplest type of pickup, with one wire coil surrounding the magnet structure. Commonly seen on the Fender Stratocaster and similar guitars. This type of pickup generally has a more trebly sound with more 'snap' and 'punch'. Generally have a lower power output. Tends to be susceptable to picking up hum from transformers and computer monitors.

Humbuckers (eg. Gibson PAF)
A bit more complex, with two coils and sets of magnets in a clever arrangement which cancels out most of the hum that the coils pick up. They are more powerful than single coil pickups, with a bassier, smoother sound. Due to the fact that you have two coils and four wires to play with, humbuckers give a variety of wiring (and thus sound) options. Some of the more modern humbuckers are made full-size but are designed to sound more like single-coils.

Variations On A Theme
In response to guitarists who did not want to change the look of their guitar, but wanted to change the sound, manufacturers have come up with a number of pickups which look like one thing but are (or sound that they are) another.

Side By Side Humbuckers (eg. Seymour Duncan 'Rails' & Di Marzio 'FastTrack')
These are humbuckers which are the same size as a single coil, They are usually more powerful than standard single coils (some are quite loud!), with more 'snap' than a full size humbuckers. Generally, the wiring options for these pickups tend to be as limited as a single coil, as wiring them anything other than series in phase gives them very thin unusable sounds.

Stacked Humbuckers (eg. Seymour Duncan 'Stacks' & Di Marzio 'HS')
These are made to look and sound very similar to a single coil, but consist of two coils wound on the same magnets. Generally, the wiring options for these pickups tend to be as limited as a single coil, as wiring them anything other than series in phase gives them very thin unusable sounds.

So if you are considering changing pickups as an upgrade option. Listen to your guitar to hear what you have already, decide what you want to achieve sonically, and do some research at pickup manufacturers' websites. And then speak to your friendly neighborhood music store (hint, hint). You may find that they will do the surgery for you (provided that it's not major surgery - but even then, they will usually have a qualified tech to send it to).