Terry,
Strictly speaking, kerf is the path a cutter leaves behind it.
However, I've always heard it used to describe how saw blades cut, in particular how the bend put in the tip of the tooth affects it. For example, a circular saw blade may be made out of steel .040" thick, but because each tooth is bent slightly outward (generally alternating from tooth to tooth), it will cut wider than 0.040". This keeps the material from binding on the disk of the blade because it cuts wider than it is. If each tooth is bent 0.015", then the kerf of the cut will be 0.015+0.040+0.015, or 0.070".
Regards