In aviation, a planform is the shape and layout of an airplane's wing and fuselage. Of all the myriad planforms used, they can typically be grouped into those used for low-speed flight, found on general aviation aircraft, and those used for high-speed flight, found on many military aircraft and airliners.
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 winglet. In a general sense all planes should use wings like those on a glider, but practical problems intrude. The primary issue is strength; it is harder to build a longer wing 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).
At higher speeds nearing the speed of sound, a new form of drag appears: wave drag. Wave drag is considerably more powerful than induced drag, and must be avoided at all costs in order to improve performance. Doing so demands a wing that is as thin as possible, with a slowly changing profile over a wide chord. Of course, this is basically the opposite goal to low speed wings, which presents a problem.
Just as on the lower speed designs, making an ideal high speed planform is difficult for practical reasons. In this case a very thin wing makes it difficult to use the internal room to store fuel and landing gear, makes the wing far less stiff torsionally, and causes increased induced drag when flying slower.
Solutions to this problem come in many forms, notably the use of the swept wing and delta wing, both of which avoid the shock wave at the leading edge. Other designs make the wing as thin as possible, leading to designs like the F-104 Starfighter.