Refer to figure.
The airflow around an aerofoil section sets up a pressure differential between the upper and lower surfaces.

The lower surface of a wing is normally at a pressure above atmospheric, whilst the upper surface is at a pressure below atmospheric. Beyond the wing tips the air is nominally at atmospheric pressure. This causes a spanwise flow of air outward away from the fuselage on the lower surface and an inward flow toward the fuselage on the upper surface. At the trailing edge of the wing, where the two flows meet, a twisting motion is imparted to the air and a series of vortices form. These are trailing edge vortices.

The effect of these vortices is to create more down wash behind the wing than up wash in front of the wing. This results in the effective air flow (EAF) being inclined to the relative air flow (RAF) by an angle called the induced angle of attack (α_i).

Because of this local increase in induced angle of attack, the overall lift generated by a wing will be below the value that would be generated if there were no spanwise pressure differential.

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.

Induced drag increases with decreasing speed (for a constant lift force). This is because, as speed decreases, the downwash caused by the tip vortices becomes more significant, the rearward inclination of the lift is bigger, and therefore induced drag is higher.