sustain is how long a note sustains for after you play it... so, when you play a note, and the sound keeps going on for quite along time until you take your finger off the fret, then that's alot of sustain, but when you play a note and even though your finger is still on the fret, the sound fades and dies out really quickly, then that is not very much sustain at all.

as for pickups, hum cancelling pickups (humbuckers and hum cancelling single coils) will help stop that humming/buzzing that you hear when you play with a lot of volume, gain, or both.

true single coils do not attempt to try to cancel that hum, and produce a brighter tone.

someone else could go into more detail on this then I could, but that's the gist of it.