Difference between Dirty Bomb and Fission Bomb - Illustration

Dirty bomb:

Explosives combined with radioactive materials
Detonation vaporizes or aerosolizes radioactive material and propels it into the air
Not a nuclear detonation

Fission bomb: one example

Caused by an uncontrolled nuclear chain reaction with uranium-235 or plutonium-239
This example has pyramids of plutonium with surrounding explosives
Initial explosion produces imploding shock wave that drives plutonium pieces inward into central sphere containing pellet of berylliu/polonium, creating "critical mass"
Resulting fission reaction causes bomb to explosde with tremendous force: nuclear detonation