"3 + 4 = 7." is known a priori.
"Chicago is on Lake Michigan." is known a posteriori.
Rationalists typically emphasize the importance of a priori ideas and arguments as the foundation of all knowledge. Kant held that synthetic a priori judgments are preconditions of experience and form the basis for mathematics and science.
Empiricists, on the other hand, usually hold that all a priori propositions are merely analytic, so that we must rely on a posteriori propositions for significant knowledge of the world. Kripke challenges even the identification of this distinction with that between the necessary and the contingent.