what is the difference between a phd and a dphil? & why do doctorates have so many different names?

There is no difference between a DPhil and a PhD. A small number of universities award the DPhil degree instead of a PhD. The result is still the same and the qualifications are recognised as being equivalent to each other. The reason for the different names arises from the historical roots of degree courses and the independence of British universities, who have the freedom to call their degrees what they like. Despite PhD being the more common term it is in fact an American concept and it may be that snobbery has kept the name, if not the format, out of the vocabulary of some UK universities.

PhD = philosophiae doctor; DPhil = doctor of philosophy. Both abbreviations are therefore equivalent in their meanings, and also in the degree for which they stand. Traditionally, many UK universities have called their doctoral degrees DPhil, while today most have adopted the US abbreviation PhD (except for Oxford and Sussex, as far as I'm aware). The MPhil degree still reminds of that time. In the US, there has traditionally been a preference for pseudo-Latin (and therefore "reversed") abbreviations, see AB, AM, PhD.