The toxicity of a chemical is often defined by its LD50-- the dose at which 50% of a population of test animals dies. LD50 is often determined using mice, which are much smaller than humans, so to make this data more useful, LD50 is expressed in grams or milligrams of chemical per kilogram of body weight.
If a mouse, for example, weighs 25g and 50% of a mouse population is killed when each mouse is given 0.1g of a chemical, the LD50 for that chemical is 4g/kg. This means that while 0.1g might kill a mouse, it would probably take around 240g to kill a human weighing 60kg.
Mice, of course, aren't exactly the same as humans, so the LD50 of a chemical isn't the final word in chemical toxicity.
Water, one of the least toxic substances known to humanity, has an LD50 of 90g/kg in rats. The estimate for a 60kg human, then, is that 5.4kg (5.4 liters). I'm not sure how bad drinking 5 liters of water is, but I don't imagine it's lethal (I suppose it depends on how quickly the water is consumed).
The most toxic substance (lowest LD50) on Wikipedia's list is botulinum toxin, otherwise known as botox, with an estimated LD50 in humans of 1 nanogram (10^-9) per kilogram. Isn't that something.