My understanding of Plato's dialogues is that, though Plato wrote them, the characters in them were real people. What I've never been clear on is whether the arguments Plato attributes to these individuals were actually their own arguments, or whether it's all just a rhetorical foil, and anything said in dialogues is actually something Plato came up with. E.g., if I want to cite Callicles' argument that might makes right, should I attribute the idea to Callicles, or to Plato?
For an exhaustive study of what we know on independent grounds about the characters who appear in Plato's dialogues, I would recommend Debra Nails' The People of Plato . In many cases, we have independent evidence that the positions that are held by the characters in Plato's dialogues corresponds to views held by the historical figures with the same names. At the same time, though, it is simply not clear how faithful Plato was to the details of their positions. For this reason, when speaking of the position that is articulated by the character Callicles in Plato's Republic , it is best to attribute the view not to the historical Callicles, not to Plato (since, such an attribution would suggest that Plato endorses this argument), but to the character Callicles in Plato's Republic .