In England, cornstarch is called cornflour, and this is a
clue to what is actually is—flour made from corn.  It’s sold in the baking section and often
comes in a 16-ounce box.  I use it as a
thickener for sauces, soups and gravies. 
Cornstarch, which feels silkier than flour, has a
tendency to clump in the box.  Always
dissolve it in cold water, stirring thoroughly to make sure no lumps
remain.  If you add cornstarch directly
to a hot liquid, you’ll end up with lots of lumps.