system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What are "depleted" uranium rounds and why are they so bad?
gpt
Uranium comes in two forms, U235, which is the fissile kind that runs power plants and explodes under the right conditions; and U238 which is far less energetic. 

Depleted uranium is mostly the 238 form, which has different useful characteristics:  uranium metal is an extremely hard, dense, and non-malleable substance.

The kind of substance that, if you put a whole lot of kinetic energy into throwing it at a target, will blast through even the thickest pieces of armor without shattering or deforming (which reduce the kinetic energy it has).   

So, a lot of depleted uranium is used in armor-piercing bullets and other projectiles, and a lot of them get fired during war.

Those bullets and warheads end up in the ground, or laying around, or embedded in the sides of people's houses, and in rivers and lakes and other watersources.  It's not just the entire intact projectile; they do chip and melt and break apart under some circumstances.

Now, just because they're "delepleted" doesn't mean they stop being uranium -- they're far less likely to cause radiation sickness and other problems, but they still experience some radioactive decay, which can potentially cause health problems for the people around where depleted uranium ammunition has been used.  Plus, since it's a heavy metal, it can have bad affects on humans like lead and mercury does.