Rails:

While riding, water is displaced from under your board. In effect it flows down the length of the board under the hull and wraps up around the rails. When this happens it wants to stick to the beveled flat surface of the rail, creating the vacuum effect that holds you in while riding.

Flatter rails with a low angle or bigger rails have more surface area exposed to this resistance, which creates more vacuum effect. As a result of this resistance these rails are slower, and turn easier. The steeper/smaller the rail, the less resistance there is, the faster you go however the harder it is to turn because there is less vacuum effect. Select your rail shape according to your desire for speed and holding power. On one end of the surf spectrum you have small gutless surf that requires more speed to perform moves and on the other end of the spectrum are the heaving pits with plenty of power to spare.

Transitional rails:

Rails that change size, angle or both as they run the length of the board have a combination of effects at different lengths of the board. However since every rider is slightly different it is difficult to get the exact rail angle/size that balances holding power and speed where the weight distribution is. These rails are more complex to learn to ride on properly but show an interesting future.

Transitional radius rails:

Rails that start off with a radius and then go into a normal rail are good to reduce turbulence where the water enters the front portion of the board. The trade off is less vacuum effect in the nose for certain maneuvers like unaided spinners (without using your hands) and sharp cutouts through the back of the wave.