What's the difference between pickups?

Hi,

Can someone explain to me the difference in sound between using a humbucker (neck position), P90 (neck position), and let's say using your neck and middle pickups on a strat?

I realize humbuckers are a fatter lower sound compaired to the single coil sound, but I was looking for a good general sound comparison description between the 3 different described pickup configs. I never tried P90s. Also would using the 2 single coils be very similar sounding to one humbucker. Thanks for any help.