The original allies were the states that declared war on the Nazi regime after the Nazi army invaded Poland in September 1939. They were the Poland, Great-Britain and France.
As the result of the quick and unbelievable expansion of Germany (France was invaded,etc.) and its domination on Europe, other states decided to intervene.
So the Allies, during the Second World War, were the countries which were opposed to the Axis powers. Among them: the British Empire (Great-Britain had been joined by some Commonwealth members (Nepal, etc), the USSR, and US (the Big Three).
They decided to join their forces and to coordinate their actions to defeat the Nazis regime and to stop its expansion.
As soon as the end of the Second World wars tensions appear between the US and the USSR, concerning the atomic bomb and the diverging political point of views. There were no direct conflicts but this eventually led to the
Cold War.