Adjectival Clauses

Hello!

I would like to pose a simple question here; although my own professor could not answer it. What is the difference between a relative and an appositive clause? How can one tell the difference (e.g. if you have to analyse a sentence containg one of these two structures - how can you tell whether it is appositive or relative?)?
I used to think that a relative clause somehow defines the sentence element before that particular clause and that an appositive clause only provides us with some additional information, but when we had to do some practical work, nobody in the class had a clue. And our professor's incapability of explaining the matter confused us even further.

Thank you,

Sara

[Tdol]

The prime minister, Tony Blair, visited yesterday- appositive- it's not defining, the two parts are the same.