Both hot cocoa and hot chocolate have chocolate as the main ingredient; however, the process of making these drinks is not at all the same. Typically, hot chocolate has a variety of chocolatey ingredients. You have to melt down the chocolate deliberately, and then add milk, cream, and sugar.
The drink is meant to be served very hot, sometimes with whipped cream on top. Hot cocoa is created from cocoa powder, which does not have a lot of fat, as it is typically made from non-fat dry milk or sometimes even just water. Also, real hot chocolate is usually much thicker and more luxurious than cocoa.