Hot chocolate" is a sweet chocolate drink made with hot milk or water, and powder containing chocolate, sugar, and powdered milk.
"Cocoa" usually refers to a similar drink made with just hot milk and cocoa powder, then sweetened to taste with sugar.
Flavored milk drinks are available in many varieties. Viscosities range from that of whole milk, to “thick milkshake” products which achieve their viscosity through use of thickening additives, sometimes in combination with freezing.
Cocoa adds color and flavor to chocolate drinks.