It was the day of Emperor Hirohito's radio broadcast to the Japanese people announcing Japan's defeat, and followed on from the atomic bombs Little Boy dropped on Hiroshima on 6 August and Fat Man on Nagasaki on 9 August. Since Japan was the last Axis Power to surrender and VJ Day followed VE Day by three months, VJ Day marked the end of World War II.
The formal Japanese surrender in fact took place on board the battleship USS Missouri on 2 September 1945.
In more recent times VJ Day is sometimes referred to as VP Day (Victory in the Pacific Day) to bring it in line with VE Day where the major enemy power was not singled out in the way VJ Day did to Japan.
See also: Japanese Instrument of Surrender