I am not sure there really is a difference. Very few seem to consider the subtle difference between them. Then again, some see them as very different... the trouble is that, though we present ourselves as a unified 'lifestyle', we are all from different backgrounds and beliefs and this means that we all have different ideas about the basic definitions.

I would say that we could also consider Master to be a title whereas Dom is a description of a role. So, for example, Master Stephen is a Dom. You would not call him Dom Stephen, though you can also use 'Master' as a description as well.

Also, while the feminine form (Domme) is often used, I have also seen the term Dom being used to describe both male and female dominants because it is more gender non specific than 'Master' and 'Mistress'