Since the chord may vary, you'll also see the aspect ratio
defined as the span divided by the square root of the wing area.

Martin Simons' book:

http://www.amazon.com/exec/obidos/AS...285061-3940604

... discusses the subject.

My potentially inaccurate (standard disclaimer ) recollection is:

High aspect ratio wings are more efficient than low aspect ratio. Tip vortices have a lot to do with this. As the air swirls around and near the tip of the wing it reduces the effectiveness of the the wing area near the tip. A high aspect ratio wing has less area, relatively, near the tip than a low aspect ratio, hence higher efficiency.

The cost is a more acute (steeper) lift versus angle of attack curve. This translates into greater pitch sensitivity and more severe stall characteristics.