Keeping fish healthy requires a constant and reliable filtration system. Three types of filtration are necessary for a typical tank: biological, mechanical and chemical. Although some filters are "dedicated" to perform a specific filtration task, most are multifunctional and can handle several – if not all – requirements. Though all types of filtration are necessary to keeping a healthy tank, experienced aquarists will agree that the biological process is the most crucial.
For more information, please read the story Selecting Filtration Systems for Your Aquarium.