Apposition is specifically, as far as I know, renaming. "The book was old, very old." -- "That guy over there, my teacher, speaks French." etc.
Juxtaposition is not a technical term in linguistics-- it's just putting two things next to each other (usually without any kind of element linking them). So for example, you might talk about asyndetic coordination as juxtaposition-- "mother father" instead of "mother and father".