Impressionism and Post-impressionism what's the difference?

Richard Shiff’s “Cezanne and the End of Impression” delves into the idea and history over the trouble of naming Post-Impressionism, just that. Similar to all the other art movements post-impressionism came as reaction to Impressionism and the rules that it had formed (even though they were breaking away from the rules). Expressionism seemed to fit “the spirit of the younger generation” (155), but ultimately post-impressionism was chosen to due chronologic order. But just like the Impressionist, post-impressionist searched to communicate their expression through their art. But Impressionist chose to “simplify and even ‘distort’ the objects they represent” (158). Cezanne seemed to embody post-impressionist value to make painting even more basic and focus more on the shapes than Impressionist. I didn’t think that paintings could get more shape basic, but as many of these articles have proven me wrong it has.
Cezanne chose to make a drastic move (at least it was seen drastic then) and made crude and distorted paintings than the previously refinement natural paintings. He chose to “go back” in a more primitive quality of painting. I understand why it was criticized as primitive now understanding the course of paintings. The Renaissance had seen moving toward a much more precise and scientific form of painting and the Impressionists completely changed that and Cezanne continued to push the limitations of art. But I admire Cezanne for doing so, for making art so fluid and allowing it to move “back” (however you chose to see it) or any direction that it is. Shiff mentions that Cezanne left paintings unsigned or without a date, in varying degrees of finished and repainted canvas with one image that had nothing to do with the previous painting painted over it. This excited me that he did not try to fit into a time period or movement, he wrote his own rules; which to me seems like a true artist. Unlike “Paris Street Rainy Day” by Gustave Caillebotte which tried to fit into the Impressionist movement, Cezanne was not trying to fit into a movement but had one created by critics to keep the paintings in order.
Although there is said to be a difference between Impressionism and post-impressionism Shiff’s argument that Cezanne had hastily applied strokes, sketchy and unfinished quality to images, does not clarify this for me. I thought that was what Impression was also, so I have difficulty in understanding the difference. Cezanne’s paintings are said to be “flat” with a uniform intensity of hue and a lack of value gradation but his images seem to mix in with the works labeled Impressionist. So what is the real difference upon first impression because I cannot see it?