What effect did the US have on WWII when it joined the war?

The United States joining WWII was probably the most important turning point in that war.  In the Pacific, it meant that the Japanese now faced a strong and highly motivated foe.  In Europe, it meant that the Allies now had a practically unlimited supply of materiel along with a large number of potential combatants to bolster them.

There was not much of a war in the Pacific before Pearl Harbor.  However, Japan dominated what war there was.  The French, Dutch, and British were much too weak in the Pacific to resist Japan.  China lacked the unity to resist and was not technologically advanced enough.  When the US entered the war, it became a major danger to Japanese ambitions.

In Europe, the Allies had been on the ropes before the US joined the war.  France was essentially out of the war so (after June of 1941) the USSR and Britain had to fight alone.  When the US joined the war, it brought a vast new source of manpower.  Perhaps even more important was the fact that the US had so many resources that it could help supply its allies with many things that they needed.  The US was a huge and rich nation and its factories were out of range of any German bombers or army units.  The US could produce vast amounts of materiel that the Allies could use.  Because the US brought so much manpower and so many resources to the war, it had a huge impact on the war when it joined.