There is quite a lot of disagreement about which birds are a distinct species or a sub-species of another species of bird. Currently, there are thought to be somewhere between 452 & 484 different species of raptor.
One taxonomical classification defines two main orders of raptors, Falconiformes (hawks, falcons, etc) & the Strigiformes (Owls). These orders are broken down as follows :
In the recent book by James Ferguson-Lees & David A. Christie, "Raptors Of The World", they split the orders of diurnal raptor differently, a full species list according to there classification can be found here.
The term hawk is confusingly used to describe all of the diurnal raptors (with the exception of vultures & the Secretary Bird). Confusion often arises from the English names of the birds, which often places birds in the wrong category. For example, the both Cuckoo Falcons (family: Aviceda) & Honey Buzzards (Pernis apivorus) are actually a types of kite. Some of the names are based on the original ideas of the bird's classification, and the birds have since been reclassified. Sometimes the name highlights the fact that the bird has characteristics of different groups, the African Harrier Hawk (or Gymnogene) (Polyboroides typus previously Gymnogenys typicus) resembles a harrier but has broadwings similar to a hawk. There is often confusion between the British & American naming of the birds, in America, many buteo's & other buzzard-like birds are called hawks, this also applies to some harriers & kites. Many of the New World Vultures are referred to as buzzards in America. Eagle owls & hawk owls are more named as a reference to their size, rather than appearance, and are owls not eagles or hawks.