Post-Impressionism was the style that developed out or reacted against Impressionism. Post-Impressionism is situated in the late 1800's and early 1900's.
Similarities between both Impressionists and Post-Impressionists are: a real-life subject, distinctive brushstrokes, thick layers of paint and vivid colors. The main difference between both styles is that Post-Impressionism aimed to attain more form and structure, as well as more expression and emotion into their paintings.
Pointillism and Divisionism both are subdivisions of Post-Impressionism. Out of Post-Impressionism developed such art movements as Cubism and Fauvism.