Drag, on the whole, is undesirable, and most aircraft design with respect to drag is intended to minimize it. Sometimes, however, it is necessary to reduce speed, and then an increase in drag, sometimes using air brakes or spoilers is useful. There are several kinds of drag. Four important ones are Friction Drag, Pressure Drag, Induced Drag and Wave Drag.
Friction Drag is due to the viscosity of the air passing around the aircraft. The friction drag coefficient remains roughly constant as speed increases.
Pressure Drag is due to a decrease in pressure of the air as it passes the aircraft. Stalling causes a large pressure drop-off in the wake of the wings, and so pressure drag is large.
Induced Drag is caused by lift. In fact, it is that part of the force due to circulation (the effect which causes lift) which acts in the direction of airflow. The higher the lift coefficient, the higher the induced drag. The section on control shows how induced drag in maneuvering limits the sustained maneuverability of a fighter. Using induced drag is another way to lose speed - by flying in a 'zig-zag' path, at high angles of attack, you can 'bleed off' energy.
Wave Drag is due to the formation of shock waves at transonic and supersonic velocities. This forms the effective 'sound barrier', whereby the energy required to pass the speed of sound is much greater than that required to make a similar gain in subsonic speed. In a fighter, wave drag can account for perhaps 3/5 of the total zero-lift (non-induced) drag. Swept wings, and specially designed wing sections mean that today's fighters have less wave drag than their predecessors.
Other sources of drag are external stores and landing gear. All of these forms of drag are simulated in TFX. "
From the manual