Low-speed
The primary concern in low speed flight is the aspect ratio, the comparison of the length of the wing measured out from the fuselage, wingspan, compared to the length from front to back, chord. Wings with higher aspect ratios, that is, wings that are longer and skinnier, have lower drag for any given amount of lift than a wing of the same area that is shorter and fatter. This is due to an effect known as induced drag, caused by airflow around the tip of the wing. As the size of the tip decreases compared to the wing's overall size, the magnitude of the induced drag is reduced.
This is why designers of gliders, who want a high lift to drag ratio, use very long and skinny high aspect ratio wings and winglets. At higher speeds, however, different forms of drag become more prominent, meaning that the large aspect ratio wings found on gliders become very inefficient. Another issue is strength; it is harder to build a longer wing with sufficient stiffness than a short one.
There are other ways to reduce induced drag, mostly by changing the shape of the wing to reduce the size of the tip. The elliptical wing found on the Supermarine Spitfire and Republic P-47 is the most efficient, but difficult to build. A practical compromise is to taper the wing towards the tip, a feature that can be found on almost all modern aircraft (including gliders).
Read more about this topic: Planform