• Pickup wiring

parallel wiring
Pickups can be wired in a variety of ways to get sounds. Most commonly they are wired in parallel which keeps the volume fairly constant no matter how many pickups are active. The "negative" wires of all the pickups are connected and grounded, and the "positive" wires go to a switch which selects the combination of pickups to be used.

series wiring

Series wiring has a volume boost when both pickups are switched on. It also tends to lose some of the high frequencies due to the resistance of the coils adding up.
out of phase wiring
Out of phase wiring has the opposite effect from series wiring. Volume is reduced because the signals from the pickups partially cancel each other out. The farther apart the pickups are spread on the guitar, the more noticeable this effect. On a 2 pickup guitar, out of phase wiring produces a nasally tone. Note that if the pickup that's wired out of phase from what's "normal" (the top one in this diagram) has exposed metal parts (like a humbucker cover), you'll get a terrible buzzing sound whenever your finger touches it. It's the same sound as when you touch the tip of your guitar cord when it's plugged into an amp that's turned on. The out of phase sound will only occur when both pickups are used. Using either one by itself will sound normal.

The diagrams simplify what it takes to implement these wiring schemes in your guitar. Some pickups have exposed braided shields that can short the wiring if it comes in contact with another shield or metal part which is very likely inside a cramped guitar body.