Membranes can be divided into biological and synthetic; the latter, in turn, are subdivided into polymeric and inorganic. There are different classifications of membranes: by the type of membrane structure they are divided into porous and nonporous (diffusion); by state of aggregation they may be solid or liquid; by morphology they are classified into homogeneous, asymmetric, composite, hollow-fibre membranes, etc.; by the type of membrane substance they are divided into polimeric, metal, glass, ceramic, zeolite, etc. The driving forces of membrane processes may include gradients of pressure, concentration, temperature or electrical potential. The main characteristics of membranes include selectivity (separation factor), permeability, chemical and thermal stability.

The concept of a "filter" satisfies the definition of a membrane, but the term "filter" usually refers to a structure that separates components of larger size (1-10 microns).