All the different catfish species can be found in the order Siluriformes; and order that contains no less than 37 different families. The 37th catfish family was actually added to the order very recently - June 2005. Almost all the different catfish species are freshwater dwellers, but in the family Plotosidae and Ariidae you will find catfish species that do not live in freshwater. Catfishes have adapted to a wide range of different ecological niches you can find small catfish species as well as huge ones like the Wells Catfish that can grow up to 5 meters (16 feet) long and the Giant Mekong Catfish that can weigh almost 300 kg (661 lbs). These two big catfish species are naturally not very suitable as aquarium fish, but there is a vast array of other species that you can keep even in a moderately sized aquarium. The Plecos are for instance very popular in aquariums and many species can be kept even by inexperienced aquarists. Two of the most popular Plecos among novice aquarists are the Suckermouth Catfish (Hypostomus plecostomus) and the Bristlenose Catfish (Ancistrus dolichopterus).

One of the most distinguishing anatomical characteristics of the catfishes is the prominent barbells found near the mouth. A barbell is a long, tactile organ that is very useful for fishes that live in murky waters where the visibility is limited. The catfish will use its barbells when hunting and manoeuvring in dark environments, just like a cat will uses its whiskers. The barbells of a catfish do however have taste buds and the catfish can therefore "taste" its environment. You can find barbells on many other fishes in addition to the catfishes, including goatfish, a few carps and some sharks.

In addition to barbells, the various catfish species share another notable anatomical feature - they are scale less. Catfish from all families but one are also equipped with a hollow leading ray that can excrete a powerful protein to scare off or injure potential threats. Some species can even harm humans and must be handled very carefully. You should be especially cautious when dealing with catfish specimens belonging to the genus Heteropneustes (in the family Plotosidae), since a sting from one of these species can require hospital treatment. There is one catfish family where the species have no hollow leading ray, the family Malapteruridae. These species can not produce any stinging protein, and will instead protect themselves by sending out a very powerful electric shock. Due to this capability, the species found in the family Malapteruridae are called Electric catfishes.

Another example of a fascinating catfish species is the Candiru (Vandellia cirrhosa). This catfish is popularly referred to as the "Vampire Fish of Brazil" since it will attach itself to the gills of other fishes and drink their blood. This fish is not very popular in Southern America since it sometimes tries to attach itself to swimming humans, when attracted to blood or urine excretion. The body of this catfish is almost transparent, which makes the fish quite hard to spot in the water. The fish is also quite small, only 2.5-6 centimetres (1.0-2.4 inches) in lenght.