When did the Cold War begin? Who was in the Cold War? When did the Cold War end?

We generally request that you ask only one question in each post.  However, since these three questions are so closely related and their answers are relatively short, I will answer all three.

The Cold War was a conflict between communist countries and countries that were not communist.  The United States was the most important non-communist country while the Soviet Union was the most important communist country.  Other countries were involved to a degree, but the US and the USSR were the most important countries.

There are no official beginning and ending dates for the Cold War since it was not a regular war.  Instead, it was just a general conflict or competition between the two blocs of countries.  However, we can say that the Cold War began in 1945, right after WWII ended.  (Some historians say it began in 1947.)  We can say that it ended in 1991 when the Soviet Union broke apart.