This is a huge question with many different aspects and many different answers for different countries.
Perhaps the main fact about the aftermath of WWII was that the US and the Soviet Union became the two superpowers that dominated the world. Just as importantly, the two became bitter rivals in the Cold War that was to last for more than 40 years after the end of WWII.
After the war, only the US and, to a lesser extent, the USSR were in any shape to dominate the world. Since the countries of Europe (as well as Japan) were devastated by the war, only the two superpowers were alone on the world stage. The fact that their political and economic ideologies were opposed to one another meant that the next few decades would be dominated by the Cold War between the two. This is the most important fact (speaking globally) about the aftermath of WWII.