The normal breakdown line between Rishonim and Acharonim, as I understand it, is the Spanish Expulsion which ended the Sephardi Mesorah. The separation line for Ashkenazim is harder to pin down.
The primary difference between Rishonim and Acharonim is mesorah. Rishonim explain Gemaras and the like on the basis of mesorah, i.e. the traditional understanding of the text as they received it from their teachers. This doesn’t mean that Rishonim don’t ever try and prove one reading as better than the other but for the most part they have an approach via mesorah.
Among the Rishonim there are various lines of tradition. So Rashi is part of one line, the Rambam part of another and the Ramban another still. Occasionally Rashi will argue on his mesorah because he feels its approach untenable for some reason. The Baalei Hatosafos were even more inclined to argue (or maybe we tend to have the arguments recorded more). Similarly, while usually the Rambam’s approach will mirror the Ri Migash, there will be occasional differences.
The Achronim look back not at their personal mesorah, but at the writing of the Rishonim and then try to determine what is right on each given issue as opposed to following a general mesorah for everything.