We hear many people referring to hot cocoa as hot chocolate, and using the names interchangably. There is a difference between the two. The short answer is that hot cocoa is made from cocoa powder, non-fat dry milk and hot water and is very low in fat. Hot chocolate is made from ground chocolate (which has cocoa butter) and made with hot whole milk and is much higher in fat. Below is a longer answer for those who want to know more about how hot cocoa is made.
Cocoa beans come from a pod grown in tropical climates, most commonly in Central America and West Africa. The ripe seed pods are split open and the beans are scooped out, then naturally fermented and dried. The fermentation is a key step to create the chocolate flavor we all love. The dried cocoa beans are then shipped to processing plants which roast and grind the beans to produce a thick paste called chocolate liquor, which has no alcohol.
Chocolate liquor is the key ingredient in solid chocolate and the precursor to cocoa powder and cocoa butter. If you take chocolate liquor, add sugar and some cocoa butter (and possibly other ingredients), and then mold and cool it you get solid chocolate. The ratio of the ingredients determines how dark or sweet the chocolate is. When you see a number that states a percentage of cocoa solids, it indicates what percent of the chocolate is comprised of cocoa liquor, cocoa butter and cocoa powder. The rest is sugar and other ingredients.
The next major step to making cocoa powder is to put cocoa liquor under high pressure which expels out the cocoa butter. This process leaves behind a dry dark cake which is then ground up into cocoa powder. Cocoa butter has almost all the original fat from the cocoa bean, while the cocoa powder generally is very low in fat content but there is small amount in it. Cocoa butter is used in many products including chocolate, body lotions and creams and it is the basis for white chocolate, which technically isn't chocolate since it has no cocoa powder. Cocoa powder is the dark, bitter and chocolate flavor that is used in baking, hot cocoa mixes, chocolates, truffles, etc. A common step is to process the cocoa powder with alkali (also called 'dutching' because it was invented in Holland) which lightens the color and flavor. The cocoa powder we use is Bensdorp Royal Dutch cocoa and it is one of the highest grades you can get.
Finally we can get to the difference between hot cocoa and hot chocolate. Hot chocolate and European drinking chocolate are made by taking solid chocolate, which as you now know has cocoa butter in it, and grinding it into a fine powder. Hot chocolate is meant to be made with hot milk which adds to its creaminess. Hot cocoa is made from cocoa powder, which has little fat, usually non-fat dry milk, sweeteners and other ingredients and is formulated to be made with hot water. So hot chocolate is often high in fat from the cocoa butter and the milk and calories. Hot cocoa is usually low fat and has far fewer calories. Our cocoa tastes very smooth and creamy even while being very low in fat. When we do tastings at stores and shows, we always get many surprised comments when people hear our cocoa is 99% fat free and made with water.
Here are some Wikipedia articles on chocolate and how it is produced - Chocolate, Cocoa Butter, Chocolate Liquor