These systems are either a single canister system or a duel canister system.  Single canister systems are most often used to control a single water quality issue.  Duel systems are designed to deal with multiple filtering requirements and sized by the number of persons in a household.  Generally the pre-filter deals with sediment and the post filter is used to remove unwanted chemicals, VOC's and foul smell or taste.  In more complex situations triple canister systems can be installed.