What does a reviewer do - is a reviewer the referee? This question is regarding math journals but I assume should apply to journals in other disciplines as well. Some journals ask you to choose a reviewer from their list (or sometimes they ask you to suggest your reviewer of choice) as you are about to submit your manuscript. My question is what exactly this reviewer does to the manuscript? Does (s)he looks at it and decides whether to send it to a referee? or (s)he is indeed the referee?
In this context reviewer and referee are basically synonymous. In mathematics I think referee is more common, but I think in some other fields reviewer is used more often. In mathematics, "reviewer" also can mean someone who writes a review of a published paper, such as on MathSciNet, but in the context of journal submissions this usage is not relevant.

In my experience though I have never seen a list of reviewers from which you can select though, only to choose the editor that handles your submission. But perhaps some journals do offer this possibility.