What are surfaces and membranes ?
By surface we mean something 2 dimensional (*). Clearly objects like a coffee cup or a pencil are 3 dimensional but their edges - their surfaces - are 2 dimensional. We can put this another way by seeing that the surface has no thickness - it is just the places where the coffee cup ends and the air or coffee begins.
We use the word membrane to mean a sheet-like 2 dimensional object, an object with area but very little or no thickness. Good examples are sheets of paper or a piece of plastic food wrap. Just like surfaces, membranes can be flat or curved; rough or smooth.