This is just a guess, but I assume a sector is an area that minecraft does all it's checks in. So the map is divided into many sectors, each keeping track of the blocks inside of it. The reason to do this, is so that the game doesn't work on the whole map at one time. Once again, this is just a guess. I'm probably wrong.
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Blargha! Blargha! Blargha!
I wouldn't listen to a thing I say. I'm probably insane.