Nuclear Fission

Nuclear fission is the splitting of atoms into minute pieces.
(See left)

Nuclear Fusion

Nuclear Fusion is the combination of multiple particles of matter to create energy.
(See right)

Uranium 235

Uranium 235 was not the original element.  This was an enhanced version of it and created much more energy than the normal Uranium.  They used complicated methods to purify the atomic structure of the ore and increase its instability.  It was used in Little Boy.

Plutonium

This was a little different than Uranium 235.  It was less powerful from the radioactivity standpoint.  And, unlike Uranium 235, it was not atomically enhanced.