Membranes can be classified into 4 categories according to their filtration properties:
As the pore size gets smaller, more pressure will be needed to start the filtration process. Surface properties also play an important role. For a hydrophobic (water rejecting) surface, more pressure will be needed than with a hydrophilic (water attracting) surface.
MF and UF are typical particle filters, whereas NF and RO change the chemical or ionic composition of the water, for instance by removing dissolved minerals. See the filtration spectrum below.