To say that Cezanne was a "postimpressionist" doesn't really say much, since the postimpressionists were all sui generis -- unique to their own interests and pursuits. (That's true of most artists, by the way, no matter what era or period or "school" they may be identified with. It is even true of the impressionists.)
Cezanne's work incorporated impressionism, cubism, even Baroque classicizing landscape by such 17th-century artists as Poussin. He is extremely difficult to characterize. The best you can do is read as many sites and books about him as you can and look, look, look -- with the closest possible attention -- at his paintings.
A number of important museum exhibitions on Cezanne have been mounted. You should seek out the catalogues.
As an art historian, I don't think any artist I've ever taught has seemed as difficult and complex to me as Cezanne. I don't place him in any one stylistic category -- he's not susceptible to that -- and I change my opinion and understanding of him each time I approach his works.
