What was the purpose of the Yalta Conference and two important decisions made there?

The purpose of the Yalta Conference was to settle the question of what Europe, especially, would look like after the end of World War II.  The major decisions made there, in my opinion, were 1) that the USSR would join the war against Japan and 2) that the US would pretty much let the USSR have Eastern Europe for its own.

The reason for this deal was that the US felt that it needed the help of the USSR to defeat Japan.  The USSR wanted a buffer zone in Eastern Europe.  So the deal was made where they got their buffer and the US got (eventually) Russian help against Japan.