Cornstarch was invented in the 1840 by Thomas Kingsford, while he was working as the superintendent of a wheat starch factory in Jersey City, New Jersey. Until 1850, cornstarch was used primarily for starching laundry and industrial uses.
Cornstarch is fine, powdery flour ground from the endosperm of corn. It's used as a tasteless thickening agent, and to give baked goods a finer and more compact texture.
It is often used for thickening sauces, as it is translucent (when mixed into the sauce), not opaque.
When using corn starch, first mix it with cold water (or other liquid) until it forms a smooth paste, and then add it to whatever you're thickening. If you add it directly into the the cooking food it may form lumps.
Outside of North America, cornstarch is known as cornflour. This should not be confused with cornmeal, which is a thicker, coarser type of ground corn (it uses the whole corn kernel), and cannot replace cornstarch in your recipes.