Refer to figure.
INDUCED DRAG
Whenever a wing produces lift, concentrated vortices form at the wing tips. The vortices are strongest at the wing tips and become progressively weaker toward the centre-line of the aircraft.

It is the production of lift itself which reduces the magnitude of the lift force being generated. To replace the lift lost by the increased downwash, the aircraft must be flown at a higher angle of attack. This increases drag. This extra drag is called induced drag.

The stronger the vortices, the greater the induced drag.

The main factors affecting induced drag are wing planform, aspect ratio, speed, and weight/lift.

The effect of planform is the principal factor affecting induced drag. The size of the wing tip vortex is directly related to the length of the wing tip chord.

A rectangular planform wing produces a much larger vortex than the tapered section. This is because the longer the tip chord, the greater the spillage of air from the lower surface onto the upper surface, and the larger the wing tip vortex.

In aerodynamic terms, the elliptical planform wing is the most aerodynamically efficient because the downwash remains constant across the complete wingspan, giving minimal induced drag.