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Drag is a force that resists an object's motion through a fluid caused by pressure and shear forces on its surface. It has two main components: friction drag and pressure drag.

Friction drag results from shear stress on the surface, depending on wall shear stress and the surface's alignment with the flow. For surfaces parallel to the flow, all shear force contributes to friction drag. 

The friction drag coefficient varies with the Reynolds number and surface roughness. In laminar flow, it decreases with the Reynolds number, while in turbulent flow, roughness increases the drag coefficient. 

Pressure drag results from pressure forces perpendicular to the surface and depends on the object's shape and orientation. 

At high Reynolds numbers, the pressure drag coefficient is generally unaffected by the Reynolds number. However, at low Reynolds numbers, viscosity influences the drag coefficient, and boundary layer separation can increase drag.

The drag coefficient, a dimensionless value, reflects the influence of shape, Reynolds number, and surface roughness. It allows findings from scale models to be applied to full-scale designs across various shapes and flow conditions.