Whole house water filters are point of entry (POE) filters, which means they are installed where the main water line enters your home and filter all the water that is distributed throughout your home. In contrast, countertop and under sink water filters are point of use (POU) filters. They attach to a single fixture, such as the kitchen sink, or are simply placed on the countertop, and only filter the water that you use in that specific place.
Whole house water filters treat every drop of water in your home. The water you use to drink, cook, shower, brush your teeth, do laundry, and clean is all filtered. Your plumbing and appliances are also protected. Under sink and countertop filters are mainly used to filter the water that you drink, cook with, and maybe run to an ice maker in the refrigerator. Under sink and countertop filters are less expensive than whole house filtration systems, but they do not protect your pipes and appliances, and only provide one source of filtered water in your home.