What does a reviewer do - is a reviewer 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.