How does streamlined shapes reduce air resistance?

[Sigmarus]

Thanks!

[imatfaal]

It doesn't always, just most of the time - it reduces "form drag" which is one of the factors of air resistance, but it can often increase "skin drag" which is another factor.  

[Sigmarus]

Erm is there any simpler but broad explanation of it?

[petrushka.googol]

Through wind-tunnel simulations the outer skin of cars and aircraft are designed to be aerodynamically optimized, with low drag and smooth air flow.