Total parasite drag is comprised of skin friction, form drag, and interference drag. Take this flat plate flying through the air at zero degrees angle of attack to the airflow. The drag is all skin friction. At 90 degrees to the relative airflow, the drag is all form drag. But what is interference drag? Slow moving traffic trying to merge with fast moving traffic on a freeway creates bottlenecks, and a lot of energy is wasted in the process.

In flight, the airflow above the wings is accelerated, so it's fast. Around the fuselage, the airflow is slower. At the junction where these two airflows meet, a wake forms behind the airplane. The wake costs energy. It also creates a greater pressure difference between the front and rear surfaces of the airplane, which increases drag. This is called interference drag. Interference drag is strongest where the surfaces meet at sharp angles. The streamlining of shapes to control local pressure gradients helps to minimize interference drag.