The whole house water filtration system filters water for the entire home. If you do not have any special contamination issues, then a whole house carbon filtration system would be recommended for general multi-purpose whole house purification. This system could be used for the water in your bathroom, shower, cleaning, and gardening.
An under-the-counter water filtration system is also called a reverse osmosis or RO system. It is the most convenient and effective method for water filtration. RO systems are the same filtration used for bottled water and were first designed to desalinate seawater. They are placed underneath the sink and store purified water to be readily available at all times. This system filters up to 99% of all contaminants that are in water including nitrate, arsenic, chromium, fluoride, radon, and other contaminants that whole house filters cannot remove. This will leave you with the purest water quality possible. They are low maintenance systems that only require annual filter change and are designed to provide purified water for a lifetime. However, it is not practical to be using RO systems throughout the entire home because it would essentially be flushing bottled quality water down the drain. This is where a whole house filtration system can be used in combination to provide filtration for the entire home.
If you have a specific problem with your water—for example, there is orange or brown staining, which indicates there is iron in your water—the whole house water filtration and the RO system have two different jobs. An RO system could treat the iron staining issue but only at the point of use drinking faucet. Whole house systems are point of entry systems that are able to protect pipes and appliances to reduce the orange staining throughout the home.