SBZ,
Crikey’s Wikipedia page sums it up fairly well. In essence there is no accepted definition of what defines or constitutes a race. If you treat one group differently from another because of your perception that they may be from a different “race” even if other people wouldn’t recognise the distinction between those groups you are still being racist! Within UK Legislation you will generally also be considered “racist” if you treat someone less favourably as a result of their skin colour, nationality, citizenship or ethnic origin as originally defined in the 1976 Race Relations Act. Religion is not included in the act (although it is in some other legislation such as inciting religious or racial hatred).
Now if only the definition of a Troll was quite so clear?