In small tanks, a filter is usually enough + some airstone for good aeration. In larger tanks, in addition to filters, powerheads are usually used. In larger tanks, while a filter is often enough to filter the water biologically, it is not enough to allow proper water circulation. For this reason, powerheads are used - they help move water and this way eliminate any "dead spots" in a tank. Alternatively, many people use 2 filters in large tanks - one at each end - to help filtration and water movement.
What filter you need (and whether you need an airstone or a powerhead) depends on the size of your tank and the kinds (and number) of fish you have. So tell us more about these, and perhaps we'll be able to help you better
