As I understand it (corrections welcome!)

Coil Spilit:

Is exactly that - a humbucker pickup is wound as two single coils connected, and the connection between them can be wired to a switch to short one coil or the other when engaged - you're splitting the pickup

Coil Tap:

The pickup is wound in such a way that after a number of turns (say 5000), a wire is taken off before the rest of the pickup is wound the say 7500 turns total. When the coil tap is engaged, only the 5000 turns of the wire will be hot, so the output is reduced.

If I'm not quite there, someone step in....

ZRP