President Truman envisioned a post-war Europe that was free and democratic whereas Joseph Stalin envisioned a post-war Europe that was divided between the communist bloc and, at least for the time being, the non-communists.
To Truman, the European countries were supposed to be able to decide their own destinies. This would, presumably, happen through free elections. This is what was supposed to happen in Eastern Europe and it was what actually did happen in Western Europe. Truman would have assumed, of course, that all of these countries would have chosen to be democratic rather than communist. By contrast, Stalin was much more concerned with the security of his own country. He did not want the Soviet Union to be vulnerable to another invasion from the West as it had been in WWII and, to some degree, in WWI (as well as in 1812 with Napoleon). Therefore, he envisioned a post-war Europe in which the Soviet Union was guaranteed control over a bloc of nations that bordered it. These countries would serve as a buffer between the Soviet Union and the West.