In August 1945 during World War II, the United states sent an A-bomb, to Hiroshima Japan, its target was the Aioi bridge but it missed and detonated over a building called the Hiroshima Prefectural Industrial Promotion Hall, this building was later rebuilt and renamed the Hiroshima Peace Memorial to honor the lives that were lost, as well as those still living.
In most parts of Japan, there are other memorial structures put in place, to commemorate the bombing and they are:

Hiroshima Peace Memorial
the Hiroshima Peace Memorial was built in 1915 by the Czech Architect Jan Letzel, the building was first named the Hiroshima Prefectural Commercial Exhibition, the name of this building was changed twice, first in 1921 and in 1933, before it was finally changed after the Secound World War to the Hiroshima Peace Memorial, to honor all who died in the Hiroshima bombing, the Hiroshima Peace Memorial is also called the Atomic Bomb Dome.
There were some controversy regarding this building, as some locals were averse to the idea of making it a memorial site, this was even when UNESCO were making moves to make it a World Heritage site.
The United States, as well as China both had their reservations too but the Hiroshima Peace Memorial was eventually made a UNESCO World Heritage Site in 1996.

Hiroshima Peace Memorial Park
The explosion of the A-bomb, left a vast open space and in April 1956, it was converted into a park, which was named the Hiroshima Peace Memorial park. Today many activities surrounding the bombing or public events that advocate for world peace, are held in the Park.

Children’s Peace Monument
Many children died during the Hiroshima bombing, some lost loved ones others suffered from radiation, to commemorate this a statue [which features a girl, with arms outstretched and a paper crane rising above her] was later erected and it was named the Children’s Peace Monument.
The theme for the statue was the story of a little girl called Sadako Sasaki, who was a survivor of the bombings but was suffering from radiation, it was said that she believed that she could be well again, if she could only make a thousand paper cranes.
Today, this sad and moving story has evolved a new custom, whereby many children and even adults send paper cranes, to the Children’s Peace Monument.

A-bomb Dome
The A-bomb Dome was part of the Hiroshima Prefectural Industrial Promotion Hall, it was the only structure standing after the A-bomb hit the building, reconstructions was not done and it was left this way to commemorate that day.

Other memorial structures is the Rest House, which was formerly a store that got caught in the bombing, only a male survivor called Eizo Nomura emerged from the wreckage, today, its part of a collection of structures, that have being kept aside in Hiroshima, as a peaceful reminder of a sad and unforgotten past.