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".
Thanks to all that replied. Been machining for almost 30 years and never knew the proper name for kerf. Been working with it for years? I can see where in some sequences it would be a good tool to use. For those of you that do not know what kerf is, it is the width of the cutter past the cut path. If that is not clear enough read some of the replies to this thread.