Is there a difference between cornflour and baking powder?

Corn starch is an ingredient in baking powder and is actually used as a thickener in recipes (by itself).

Corn starch is the starch of the corn grain obtained from the endosperm of the corn kernel. It is used as a thickening agent in soups and liquid-based foods such as sauces, gravies, and custards.

Baking powder is a dry chemical leavening agent used to increase the volume and lighten the texture of baked goods such as muffins, cakes, scones, etc. Most commercial baking powders are made of an alkaline component (sodium bicarbonate) and inert starch (cornstarch) and one or more acid salts.