An air pump helps oxygenate the water. The pump is connected to the electrical supply and is usually a black box that sits behind the aquarium. It should never come in contact with the water because it can be dangerous. You can have a variety of air stones or ornamental ornaments that help circulate the oxygen in the tank. You will also need some sort of filtration system.
Filters remove waste matter and help keep the water pure for months at a time.
There are mechanical filters which have become more common. The most common mechanical filter and the cheapest is a square transparent box fitted inside the aquarium with an airlift. There should be a lid on this type of filter and inside the box there should be nylon floss. After you connect the filter to the air supply, the water is filtered through the nylon floss. There are also biological filters.