There are two main types of pond filtration. Mechanical filtration which removes the larger debris and Biological filtration that looks after the water quality to keep the fish healthy and happy. Most modern filters combine these methods into one unit for convenience and ease of installation.
It is a common misconception that clear water is healthy water, and most beginners believe the sole job of a filter is to remove muck and debris from the water. The truth is not the case, although a small amount of suspended particles are strained out by the biological filter a combination of both types is necessary for optimal conditions.
A mechanical filter, usually matting, a sieve or some sort of strainer physically removes particles only.
A biological filter works like a mini sewage system for the pond. In simple terms the media within the filter creates a huge surface area, which becomes home to many millions of helpful bacteria.
It is vital to keep these oxygenated by passing pond water through the filter 24 hours a day.