differences between, performance review contributors/multi-rators and approvers

Content

I would like someone to explain to me the distinctions made between, performance review contributor, Multi-rater and Approver.

My understnading is that the contributer can read the managers scores on the employee and make comments e.g. where do those comments go?  Do they permently get attached to the managers's employee (review form)?  Does the employee see those comments?  Does an trigger e-mail exist (like for the multi-rater scenio) for when you want someone to be a contributor?  I'm assumign the contributor cannot approve the form just amke comments on it. 

I can go on but I think just answering the distinction of the three would help me in making decisions on what to use