Drag
Drag is the force that resists movement of an aircraft through the air. There are two basic types: parasite drag and induced drag.
Parasite Drag
Parasite drag is comprised of all the forces that work to slow an aircraft’s movement. As the term parasite implies, it is the drag that is not associated with the production of lift.
Most of this drag is form drag, which is generated by the air being forced around the shape of the aircraft.
Parasite drag increases with the square of the airspeed.
Induced Drag
Induced drag is drag generated as a by product of the lifting force.
Whenever an airfoil is producing lift, the pressure on the lower surface of it is greater than that on the upper surface (Bernoulli’s Principle). As a result, the air tends to flow from the high pressure area below the tip upward to the low pressure area on the upper surface.
This causes a vortices that spirals or mixes at the wing tip. This spirally air is actually providing a slight downwash of air on the main wing (as if the wing were flying in an air mass moving slightly downward).
This causes our lift force vector, instead of pointing straight up, to move slightly rearward. The horizontal component of this vector (the part pointing backward) is our induced drag.
Near the ground, these vortices are disrupted by the ground, and this downwash is reduced. This reduction is induced drag near the Earth is what we call "ground effect".
As you fly faster in level flight, the wing produces more lift and thus need a lower angle of attack to maintain the same lifting force upward. As the angle of attack is lowered, the vortices become weaker, and thus induced drag is lowered.
Induced drag dissipated with the square of the airspeed.
Total Drag
Where the total drag is lowest forms the best glide speed.