Yes, the change from not being pressed into your seat to being pressed into your seat is jerk. Basically the difference between two acceleration "levels".

Snap is how abrupt those changes between accelerations are. If you accelerate a bit, and then suddenly the light turns orange and you floor it, and the turbo kicks in, and your passengers go "woaaaaaah going to need a barf baaaaag" for example, it was probably not very smooth.

Sometimes instead of linear accelerations, those concepts can be easier to understand as changes in angular motions. You're on a curve of a certain radius, and suddenly the radius changes. The change is the jerk. How sudden the change happens is the snap.