U-235 is fissionable. U-238 is not.
Enriched Uranium has a higher-than-natural level of U-235. The more highly enriched the Uranium is, the more U-235 it contains, and the more potentially dangerous it is. Moderately enriched Uranium is generally used for domestic power generation. Highly enriched Uranium is needed for nuclear weapons (and nuclear fuel for subs, carriers, etc.)
Enrichment of uranium is done by diffusion (old technology, still used in the US and France), centrifuge (prototype only in the US), and laser (high tech).
Depleted Uranium (99+% U-238) is about as dangerous as Lead but heavier.