1. Straight fins for racing (slalom, speed, or Formula). Not very user friendly
2. Very curved (swept back) fins for waveriding. Good maneuverability.
3. Freeride fins, curved but closer to 1. More user friendly than 1.
4. Crossover, freestylewave, bump/jump, etc fins. A little straighter, and better for blasting than 2.
A lot of combinations of the above.
If you have too small fin under your board, you will go fast (once planing), but you'll have trouble:
-getting on a plane
-with spinouts probably
If you have too big fin, you'll plane easily and point well, but you'll also have:
-lower top speed
-control problems, board tipping over, tailwalking, etc.
Edit: is this a windup? ;-) Sorry if it's not.