Toons are a species of artificial, living beings made out of magical paint.
If drawn with Magical Paint by trained animators, in a technique pioneered in the 19th century by inventor Victor von Duckenstein, caricatures and cartoons can be made to come to life as fully sentient beings, known as Toons. (It is also possible to create "Toon objects" using magical paint, though those have a dismaying tendency to come to life as well.) The Toons can then act as the stars in animated cartoons that are thus much cheaper to make than drawing them frame by frame by hand. Though some Toons are purely imaginary, others are created as doubles of preexisting people, in order to create cartoons about those people; there is thus a toon version of Mickey Mouse.
Toons' personality may or may not befit their physical appearance; that seems to be largely random (costars Baby Herman and Roger Rabbit are at opposite ends of that spectrum, for example). At any rate, Toons are most famous for not following conventional physics; instead, owing to their cartoon-based, supernatural nature, they abide by the rules of comedy. For instance, they are capable of walking on thin air — but only at times when it is funny to do so — and easily survive getting flattened, crushed, incinerated and other such "amusing injuries". They also do not age. The one way to kill a Toon seems to be using Dip, a special sort of paint thinner, which dissolves them — though even then, repainting the Toon back to life is possible, and will truly resurrect the deceased (complete with all memories) rather than create a doppelganger as one might expect.