Humbuckers are essentially two single coils wired together. This cancels what is called "60-cycle" hum. Single coil pickups tend to have a hum that goes along with the switch positions that select only one pickup at a time. In the early Strat days the Strats only had 3 pickup positions... leaving every position with a nice hearty hum... which really has become part of the Strat sound... Gibson introduced the "Humbucker" when they realized that if you hold the Strat pickup selector inbetween any two of the 3 positions... it would select two pickups... thus cancelling the hum. Gibson took it a step further and created the Humbuckers we know now. The tone is generally fatter... with a wider range of frequencies... You can get into a lot of detail when it comes to describing the tone of humbuckers... whether they're active or passive, what kind of magnet they have (alnico IV, alnico V,or ceramic)... and what guitar they're on. All those things influence the sound of the pick up itself. Some humbuckers sound very similar to single coil pickups... but without the hum... depending on how you wire a humbucker you can have the ability to go in and out of vintage and modern tones by coil tapping... coil tapping is when you wire a kill switch to one of the single coils that make up a humbucker... this allows you to cut the signal to one of the coils... giving it a nice strat'ish tone with hum cancelling ability... or engeage both coils and have a nice modern tone. Some humbuckers... like the EMG 60, 91, and 85s are active... which means they need a battery powered preamp to run them... this makes them very high output... great for distortion... put the fact htat they have ceramic magnets on top of that... and that makes them that much hotter (high output). They get great distortion in any application... but they don't get great vintage tones. And when it comes to vintage strat tones... their is no substitute for a good Fender single coil pickup set. You can use the search feature on this site to search the thread about pickup magnets.... or humbuckers... or single coils... and there should be some more good info.