Ultimately, it was President Harry Truman who ordered the dropping of the both atomic bombs. Obviously there would have been a chain of command, but this wasn't a decision made by someone on the frontline randomly.
General George C Marshall was head of the US military at the time, so must also bear some responsibility.
The bomb dropped on Hiroshima (from the Enola Gay) was known as "Little Boy".
The bomb dropped on Nagasaki three days later was significantly larger and was known as "Fat Boy".