Rails are arguably the most important factor when it comes to control and feel of your board. The fuller the rail the more float you will have due to more foam under your feet. However, all boards will have multiple types of rails as you go from nose to tail it will go softer to boxier. You kind of have to mix and match to get what you ultimately want.

Boxy:(most foam) Good for smaller days to help drive you down the wave because they will dig into the wave. Also they will help you make snappier turns, almost sliding across the water. But, they are hard rails to sink so heavier footed surfers these rails might be your best bet.

Mid: This rail has the rail meeting the bottom of your board further underneath your board, this will make the board a little looser while keeping the float.

Rolled: A very soft forgiving rail, good for eggs, hybrids and your longboard.

Low(round/doomed): (Least foam) You can sink this rail incredibly easy allowing for sharper turns.

Comments:
   If you are having problems spinning out on your bottom turn or, when you lean to hard on the rail it could be because the hard rails in your tail come up to high on your board creating a pivot point, where the board actually pivots the fins out of the face of the wave.