What are the differences between laminar and turbulent boundary layers?

Laminar boundary layers have smooth, orderly flow with lower skin friction drag but are prone to separation in adverse pressure gradients. Turbulent boundary layers have chaotic, mixing flow with higher skin friction but are more energetic and resist separation better. Transition from laminar to turbulent depends on Reynolds number, pressure gradient, and surface roughness. Aircraft designers often want laminar flow for reduced drag but turbulent flow where separation resistance is needed.