The key difference lies in control of the nuclear fission chain reaction. In a reactor, the reaction is controlled using moderators and control rods to sustain a steady energy release. In an atom bomb, the reaction is designed to be completely uncontrolled, creating a massive, instantaneous explosive release of energy.

Final Answer: $\text{In nuclear reactor, a controlled chain reaction of nuclear fission takes place, while in atom bomb uncontrolled chain reaction takes place.}$