Surfaces have different uses, including producing new film or layers of material and being a base to which another substance is bonded.
A typical substrate might be rigid such as metal, concrete, or glass, onto which a coating might be deposited.
[1] Some substrates are anisotropic with surface properties being different depending on the direction: examples include wood and paper products.
With all coating processes, the condition of the surface of the substrate can strongly affect the bond of subsequent layers.
[4] A substrate may be also an engineered surface where an unintended or natural process occurs, like in: This technology-related article is a stub.