Mechanical filter media is the first level of the process, waste food, debris and dirt particles are trapped and removed from the water. It takes the form of pads, sponges or wool. Various types are usually put together, such as coarse and fine, mean that the water is forced through different types of material and ensures that all particles of waste are removed.
Biological filter media uses the bacteria from the waste products to form healthy, helpful bacteria that in turn provide filtration. Using ceramic biological media gives bacteria a surface area on which to develop and grow. These cultures encourage constant biological filtration.
Chemical filter media is usually the last stage of filtration. Placing it at the end of the process means that none of the helpful bacteria on the biological media is removed. Carbon is often used as a chemical filter, as very fine particles stick to it, leaving the water as clean as possible.
While aquarium filters do an excellent job at removing any unwanted waste and debris, there are other factors which can affect your tank and cause problems such as nuisance algae. Various treatments and additives are available to tackle such problems, and so it's worth having a selection of these in your cupboard ready and waiting.