World War II officially ended with the formal surrender of Japan on September 2, 1945 after the U.S.A. dropped two atomic bombs on the country, which killed over a 100 thousand people. The Allies offered Japan a chance to surrender prior to dropping the bombs, but they refused. Germany had already surrendered to the Allies in May.
Unofficial unconditional surrender came a few weeks prior to the signing of the official surrender. The term "V-J Day" applies to the August 14 or 15 unofficial surrender and also to September 2 and the official surrender. V-J Day or "Victory over Japan day" is celebrated in a number of locations in remembrance of the war and everyone that died.