The shape of your board's edges has a profound impact on the tracking capabilities of the board.  The sharper the edge or rail, the more aggressively the board will track, resulting in improved acceleration and overall speed.  The drawback of a sharp edge is that it is easier to catch an edge, resulting in a fly swatter like effect with your face.  A sharp edge is less forgiving than its rounded counterpart.  Beginners and surface tricksters are well advised to look for boards with rounder or buttery edges.  A common trend emerging these days is the use of a variable edge in intermediate to advanced boards.  In the past these boards tended to have a sharper edge for harder, faster turning.  However, as tricks become more technical and the demand is growing to have a looser feel to the board among the top riders manufacturers have started varying the edge thickness and shape to provide this.  The result is an edge that tends to softer (rounder) in the middle of the board allowing a rider to slide, or butter, the edge more easily, and towards the tip and tail the board the edge grows progressively sharper in order to maintain that high level of tracking or grip while edging.

This works surprisingly well because of the shift in rider weight on the board when doing different things on the water.  While doing surface tricks the rider tends to be relatively balanced on the board and thus using the middle part of the board’s edge (where it’s more rounded).  When edging hard the rider’s weight tends to shift towards the back foot in order to avoid digging the tip, thus putting the weight further back on the board and using the edge towards the tip or tail, where it is sharper and will carve harder.