I'll take a stab...
The hit is describing an event in a good swing that is the culmination of many forces coming together in unison (i.e. feet, legs, weight shift, hips, torso, core, shoulders, upper arm, lower arm, wrist, pivot). In order to really throw far, consistently, and dependably you must be able to control the hit. By control the hit I mean be able to cause it in varying degrees from barely to full blown crush. It is called hit because, well, it is what it feels like. Imagining that you were going to strike someone in anger in a similar fashion as you throw a disc, it would be the point that you make contact with the recipient of said aggression.
Snap refers to a few things:
Snap 1: Snap is the amount of torque applied to the disc. Snap can refer directly to the numerically ascertainable (through observational science) number of rotations. It is the rotational force applied to the disc at the time of the "hit."
Snap 2: Snap can also refer to the ability of one to "hit." i.e. that guy has great snap. This doesn't necessarily mean that he spins the disc fast, but rather that he "hits" well.
Snap 3: The audible sound when a disc is released. There is a difference between "snap" and "slip" and even more dispute over whether hearing snap is good, bad, or neutral.
Snap 4: The physical act of snapping the disc. (Very similar to Snap 2) This refers more to the mechanics in themselves as opposed to the form as a whole. Primarily wrist pivot only, and more to do with controlling the Snap 1 type rotational force.
Long story short, snap can be equivalent to hit, snap can be produced by the hit, snap can be one aspect of the hit.