Filtration by activated carbon is based on a physical phenomenon called adsorption. Molecules in a liquid or gas stick to the surface of an adhesive material as they pass across it. All solids are adsorbent to some extent but two factors greatly influence a solid's ability for adsorption; surface area and attractive force. Activated carbon is an extremely porous solid which gives it a large, mostly internal surface area. This area ranges from 500 up to 1500 square meters per gram. This means there is lots of surface to attract and hold molecules.

Carbon is obtained by burning raw materials such as wood, coal, peat and coconut shells. Carbon becomes activated by heating it with steam to approximately 1000 °C in the absence of oxygen (O2). This treatment removes residual non-carbon elements and produces a porous internal microstructure having an extremely high surface area. Activated carbon is an amorphous form of carbon. This means that it has no regular atomic structure, unlike the other forms of carbon like diamond and graphite. Activated carbon differs from the familiar charcoal that is used in barbecue grills. Common charcoal contains other organic residues, is much less porous, and has a lower surface area.
Dependent on type of application, three major product groups exist:
Powdered activated carbon; particle size 1-150 micron (PAC)
Granular activated carbon (granulated or extruded), particle size in the 0.5-4 mm range (GAC)
Solid block carbon created by compressing finely pulverized activated carbon with a binding medium and fusing them into a solid block