They both mean the same thing, but ''προτού'' can only be used before a verb (in simple subjunctive). Eg: πριν φύγεις / προτού φύγεις = before you leave. But, if you want to say, for example, ''three years ago'', you can only say ''πριν (από) τρία χρόνια''. So ''πριν'' can stand alone as ''ago'' and it can also be used before a verb :)