There are two recognised scientific families of owls. The Tytonidae (Barn, Grass and Bay Owls) and the Strigidae ("true" owls). The Strigidae contains many more species and basic descriptions of these are outlined below.
These are the Barn and Bay Owls. They are found in temperate and tropical areas throughout the world.
These are the rest of the owls including species of Scops owls, Eagle owls, Fishing owls, Wood owls and Eared Owls.
Main groups of Owl genus within Strigidae family:
Otus- Scops and Screech Owls
There are 67 species in this genus and there are species found in America (Screech Owls) and Europe, Asia and Africa (Scops Owls). All species are tiny (16-28cm) and are mostly insectivourous. Many species come from restricted areas (islands mostly) and some are classified as critically endangered in the wild.
Strix- Wood Owls
Hence their name, these owls tend to live in woodland areas. There are 20 species in the Strix genus and 4 species in Pulsatrix which are wood owls from Central and Southern America. They include our native Tawny Owl and others such as Asian Brown Wood Owl and Great Grey Owl. They are medium to large sized owls with rounded heads. Eyes are relatively large to allow more light to enter in subdued lighting of woodland. Plumage is mainly cryptic, with several species having colour morphs- including native Tawny Owl.
Bubo- Eagle Owls
20 species in this genus and are generally large, with prominent ear-tufts and powerful talons. Most are large and very powerful. The European Eagle Owl is the largest species of owl in the world. Species in the group are found in Europe, Asia, Indonesia, Africa and America. The Snowy Owl is a very close relative of this group, but not included. Some of the fishing-owls are part of this genus.
Asio- Eared Owls
These owls are the same as the Eagle Owls (Bubo) in having ear tufts. There are 7 species recognised worldwide. We have two native species: the Long-eared and Short-eared Owls, but both are quite rare. The Short-eared owl lives in many parts of the world and do especially well on islands where there is little competition with other species.
Athene and Aegolius- Small Owls
There are 4 species of Athene including our native Little Owl and also the American Burrowing Owl. There are also 4 species in the Aegolius genus which include the Tengmalm's Owl in Europe.
Scotopelia- Fishing Owls
This genus has only 3 species but some of the fishing owls are within Bubo genus. All species originate from Africa. They are medium to large sized owls without ear-tufts. Talons are long and curved, soles of feet have spicules (as have fish-eating eagles to help hold on to slippery, wriggling fish). This group is only found in Africa, south of the Sahara, and are very close relatives of Bubo genus, in which other fishing owls are found.
Glaucidium- Pygmy Owls
Very small to tiny owls with rounded heads, no ear-tufts, and yellow eyes. There are 30 recognised species from America, Europe, Africa and Asia. One of the most common in this group is the Eurasian Pygmy Owl from central and northern Europe found mainly coniferous montane forests.
Ninox- Hawk Owls
These originate from Eastern Asia and Australasia, with one species found in Madagascar. 20 species are recognised with the common Australian species being known as the Boobook from the call it makes. More species may exist as some considered sub-species could be raised to species level with further research and DNA work.