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What is Drag? On a flying aircraft, drag is the force working in opposition to the thrust force. Streamlining a body is to minimize the amount of drag a body produces. It is important that engineers streamline aircraft, in order to increase the overall aerodynamic efficiency and aircraft performance. 
There are essentially three types of drag: friction drag, form or pressure drag, and induced drag. Each type of drag contributes to the overall drag force. Friction and Form Drag Friction drag occurs at the boundary layer. This is the area close to the airfoil of the plane where the average speed of the airflow is slower than the surrounding air. This "skin" of air is what slides against the surrounding airflow and causes friction. In the boundary layer, the flow can be laminar or turbulent. A uniform flow of air, parallel to the airfoil, is the laminar portion of the boundary layer, found closer to the leading edge. A turbulent portion of the boundary layer, near the trailing edge, is irregular and disoriented. A boundary layer that is turbulent increases the friction drag. Somewhere between the leading edge and the trailing edge, the boundary layer changes from laminar to turbulent. In order to bring down the friction drag, engineers try to delay this change as much as possible. One way of achieving this is to keep the airfoil as smooth as possible. The smallest of irregularities, even tiny bugs that hit the airfoil, can have the overall effect of changing the boundary layer to turbulent sooner. 
The viscosity of the fluid is directly related to the friction drag a body experiences when it goes through it. Although, air has a relatively low viscosity, the friction drag is a significant contributor to the total drag of an airplane. It becomes even more prominent when the wing area is relatively large. Form drag is another major portion of the total drag in most airplanes. This type of drag is directly proportional to the cross-sectional area of the object traveling through a fluid. To create a better mental picture, imagine sticking your hand outside the window of your shiny sports car. If you turn your hand so that your palm is facing the wind, then that is considered as the form or the pressure drag. The airflow hitting your hand slows right in front of you, then it speeds up as it passes your hand, and becomes turbulent afterwards. This creates a situation where the air pressure in front of your hand is higher than the air pressure behind it, leading to a net force pushing downwind. A force acting perpendicularly to the face of an object produces form drag. Relationship of Friction Drag to Form Drag in Air Let's change that mental image by letting your palm face the ground. First of all, congratulations, you have just streamlined your hand. As a result, you feel a lot less drag. However, there are still some drag forces acting on your hand. The cross-sectional area of your hand has decreased a whole lot, so you feel a lot less form drag. This is the reason why you've felt such a sharp decrease in the drag. However, the area parallel to the airflow just became longer, which means the friction drag just increased because there is more of your hand that the air has to slide over. The net drag you felt is substantially smaller than in the previous instance, which illustrates the point that in this case, the form drag accounted for a lot more of the total drag force than the friction drag. Induced Drag Finally we come to induced drag, or drag caused by lift. Using the same hand out the window example, if you slightly tilted your hand so that your palms were facing forward, yet still down, then you experience some upward force, lift, yet you also feel a drag force. This is the reason why engineers also call this the drag due to lift. 
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