Guitar Pickups

This device converts string vibrations into electricity, and is embedded in the body of the guitar right beneath the strings. ... Six magnets are used to better pick up the sound from the six stings. Some pickups use metal rods instead of magnets.

The bridge pickup produces a brighter tone than the neck pickup, which sounds warmer and more mellow. The bridge is often used for lead and heavier styles of music with more gain such as rock and metal, whilst the neck is used for cleaner rhythm tones.

Yes, they make the single largest amount of modifiable difference to your sound. While choice of amp will be the largest difference, followed by speaker choice, the difference between 50's strat single coils and hotter 90's strat single coils is vast.

You're fine. Unless your trying to fit an Active pickup battery pack into a guitar, or trying to fit a humbucker in a single coil slot, you can put any pickup you want it.

Single-coil pickups tend to have brighter, crisper tone. Comparitively, humbuckers typically have what guitarists describe as a “thicker” sound; one that is perceived as rounder and warmer.