There were two types of atom bombs created one was called the "Little Boy" and the other was titled the "Fat Man."
The Little Boy
The "little boy" atom bomb was designed to be a gun type bomb. They call it a gun type bomb due to the fact that the bomb shoots a mass of uranium 235 into another mass of uranium 235 to create a supercritical mass.
www.atomicarchive.com/Fission/Fission6.shtml
Once the supercritical mass is formed the initiator emits a burst of neutrons creating the fission chain reactions. These chain reactions continue until the energy released becomes so big that the bomb blows itself up.
www.cccoe.k12.ca.us/abomb/physics.htm
The Fat Man
www.atomicarchive.com/Fission/Fission8.shtml
The Fat Man atom bomb was created due to the fact that plutonium 239 has such a fast spontaneous fission rate. A new design was needed to facilitate such an isotope. The Little Boy is too slow to work with such an isotope. So, Seth Neddermeyer created an idea to use explosive charges to compress a small sphere of plutonium quickly to a required density that will make the plutonium go critical and produce a nuclear explosion. Thus, the Fat Man atom bomb was born.
