Since you are likely asking about both depleted Plutonium and depleted Uranium, I will give you data on both.
Uranium is a naturally occurring weakly radioactive mineral that is used to fuel nuclear reactors and is the primary component of Nuclear Weapons.
Uranium, like most elements, come in various "flavors," if you will, called isotopes. Isotopes refer to the fact that many elements can exist with more or less than the normal amount of neutrons in them. Uranium's natural weight is 238, but there are U234 and U235 isotopes, that is, a small portion of uranium atoms have three or four fewer neutrons than normal.
One of the differences between U235 and its common relative U238 is that U235 fissions very easily. Fission is the process of "splitting" an atom, releasing large amounts of energy, mostly in the form of heat.
Unfortunately, U235 is relatively rare (approx. 0.71% of natural Uranium ore) so the uranium ore is processed to provide a mixture that has more of the U235 isotope in it (around 4%). This is called "enriched uranium." The byproduct of this processing is U238 with almost no U235 in it at all, and that is "depleted uranium."
U238 in and of itself is not very fissile. When bombarded by neutrons released by U235 fission, it absorbs neutrons to become Pu239-- Plutonium. The Pu239 isotope of plutonium is fissile, and works even better than U238. It occurs very rarely in nature, and is mostly produced in nuclear reactors as a byproduct (or in so-called breeder reactors designed specifically to produce plutonium) and is used almost exclusively in nuclear reactors and nuclear weapons. Plutonium is also used in some spacecraft (explorers travelling outside earth orbit) as a power source.
Plutonium's primary radioactive decay product is alpha rays. Alpha radiation cannot penetrate a sheet of paper, and human skin is more than enough protection against it. If ingested, breathed in, or if plutonium gets into the blood stream through a wound, then the alpha radiation can cause damage to DNA and increases an individual's chances of acquiring cancer. When in the blood stream, plutonium will settle in the liver and bones.
Depleted Uranium is 40% less radioactive than natural uranium and, like plutonium, emits primarily alpha radiation. Because it has 1.7 time the mass of lead, depleted uranium has been used as projectiles in certain types of weapons. The additional mass provides more kinetic energy to the projectile and therefore has more penetrating power when used against armor (the A-10 Warthog aircraft houses a 30mm cannon using depleted uranium rounds as an anti-tank weapon).
Depleted plutonium contains 19% or more of the heavier Pu240, Pu241and Pu242 isotopes, the even isotopes are not very fissile. P241 emits low-energy beta radiation (clothing is typically sufficient to protect an individual from beta rays) to become Americium241, which emits gamma radiation at a much lower rate. Gamma radiation will penetrate deeply in to the body. Because Pu241 has a half-life of around 14.4 years and Am241 has a half-life of 432 years, the older a sample gets, the more dangerous it becomes as a radioactive material.
Contrary to popular (in some circles) belief, depleted plutonium is not used in weapons, it is too radioactive for conventional weapons and too unpredictable in nuclear weapons. But it is far from being "the most dangerous substance known to man," as there are other, more common non-radioactive materials that can kill a person a lot faster than exposure to depleted plutonium.
The is no such beast as 'depleted plutonium'. The substance I believe you're after is 'depleted uranium', which is used in weapons projectiles and has been in the news recently over alleged health complications in Kosovo and the Persian Gulf.
D. Paradis, Avionics Instructor, NAMTRAU Lemoore, CA
Depleted uranium is produced during the recycling of spent fuel rods from nuclear reactors, during which plutonium, thorium and the most radioactive isotope of uranium (U235) are recovered for re-use in new fuel rods. The remaining 'depleted' uranium has very low levels of radioactivity - in fact, less than the original unrefined uranium oxide dug out of the ground to produce the fuel rods in the first place.
Uranium is a wonderfully dense metal - nearly twice as dense as lead - meaning that a given volume of it (say, an artillery or tank shell) packs more wallop than other metals. This is obviously attractive from a military standpoint. The problem arises not from the radioactive characteristics of the depleted metal but its chemical properties. When DU-tipped armament explodes the uranium can be rendered as very fine dust, easily ingested or breathed in, especially by children playing in burned-out wrecks. Uranium is chemically toxic, but not in low concentrations. However, there are unknowns, for example, about the long-term risks from ground water contaminated by buried DU shrapnel.
See UNEP/UNCHS' report of the environmental impacts of DU usage in Kosovo at http://www.grid.unep.ch/btf/final/
Mick Wilson, B.S., Environmental information systems manager, Nairobi