A body moving relative to a fluid experiences a force component opposite the relative motion called drag.

At moderate and high Reynolds numbers, drag is commonly summarized by

$$D=\frac12\rho v^2 C_D A,$$

where $A$ is a chosen reference area and $C_D$ is a dimensionless drag coefficient that depends on shape, orientation and flow conditions.

Pressure drag

Pressure is not distributed symmetrically when the flow separates and forms a wake. The resulting front-to-back pressure difference produces pressure drag.

Bluff bodies tend to have strong separated wakes and substantial pressure drag.

Skin-friction drag

Viscous shear stress acts tangentially on the body surface. Integrating this shear over the surface produces skin-friction drag.

Streamlined bodies can have lower pressure drag while exposing more wetted surface to skin friction.

Drag coefficient is not universal

$C_D$ can change with Reynolds number, surface roughness and flow regime. The drag equation is therefore a useful scaling form, not a claim that each shape has one fixed coefficient under all conditions.