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As you could discover from looking in a proper chemistry textbook or even, god forbid, wikipedia[wp], thallium is a metallic chemical element. It is not particularly abundant in the earth's crust, neither is it particularly useful. It is however, particularly poisonous, and serves as a howling example of scientific and technological idiocy.
As recently as 1860, nobody had the faintest clue that thallium even existed. However in 1861, a couple of guys discovered it and scientists began exploring its properties. One of these was that thallium compounds tended to be exceedingly poisonous.
(This is largely because thallium forms large flabby singly-charged ions which readily dissolve in water, and have a similar surface-charge density to the vital nutrient potassium. Your body cells thus mistake thallium for potassium, and readily absorb it. However beyond surface-charge density, thallium's chemistry is very different to potassium: like most metals in its region of the periodic table, it is highly "thiophilic", meaning it binds to sulfur atoms wherever it finds them, wrecking biomolecules containing sulfur... nasty).
In any case, dumb people decided to use thallium compounds as rat poison and insecticides. It worked brilliantly. The problem was, it killed people too. Doh. One of the symptoms of thallium poisoning was hair loss. Can you guess what facepalmingly stoopid commerical use for thallium this led to? Yes, you guessed right: a depilatory cosmetic (in other words, hair removal cream for ladies). And it worked brilliantly - that hair fell right out. Hairy legs were banished. There was just one small problem: people tended to also die. Still, they probably looked gorgeous in their coffin.
Thallium compounds are usually tasteless, and so when it also became a weapon of choice for poisoners, as it could be slipped into food or drink without the victim noticing.
For other examples of this kind of "gee, what can we do with this new element?" foolishness, see radium.