Absolute means absolute. An absolute 5-micron filter is one proven to remove virtually all particles of five-microns or larger.
Axial Flow Cartridge: A filter cartridge in which water enters on one end, flows the full length of the cartridge, and exits through the center hole at the other end. Axial Flow cartridges usually contain granular or loose-fill filter media.
Carbon Block: Filters in which the carbon is pressed or molded into a tight block-like structure. Generally speaking, carbon block filters are more effective than GAC filters. Carbon block filters are radial flow filters. (See below.)
Ceramic is used to make very tight filters whose absolute filtration rating is too tight to allow the passage of bacteria or pathogens like cryptosporidium or giardia. Ceramic cartridges "purify" water by straining out pathogens. They do not remove chemicals (unless accompanied by an additive, like carbon, that does).
CTO: Stands for chlorine/taste/odor reduction. Filter makers use this designation for filters for which the maker only claims aesthetic improvement (although the filter may well do much more than promised).
GAC: Granular activated carbon. Loose-fill filters in which the carbon is essentially the consistency of coffee grounds.
Gravity Filter: A filter that works without water pressure. You pour water into an upper chamber and it filters through to a lower containment vessel.
KDF is a metallic (copper/zinc) filter medium used to remove free chlorine, lead, iron, and hydrogen sulfide. It is usually mixed with granular activated carbon. The treatment process is usually referred to as "redox," which describes the interaction of dissimilar metals.
Nominal: means "more or less." A nominal 5-micron filter is one that removes 85% or so of 5-micron particles.
Radial Flow Cartridge: A cartridge in which the water flows from outside to inside and uses the entire surface area of the carbon. Carbon block cartridges are radial flow filters.
Sediment Filter: A filter designed to strain out particles. Sediment filters normally do not remove chemical contaminants.
Siphon Filter: Water is "sucked" from an un-pressurized container, through a filter, to a vessel located beneath it. Siphons can provide much better flow rates than standard gravity filters.
(whole house & well units)