The primary difference between enriched and depleted uranium lies in their isotopic composition. Enriched uranium contains a higher percentage of the U-235 isotope, which is needed for nuclear fission in reactors and weapons. Depleted uranium contains a lower concentration of U-235 and is often a byproduct of the uranium enrichment process. Depleted uranium is denser and can be used in military applications, while enriched uranium is essential for power generation and nuclear weapons.