Many people use hot chocolate interchangeably with hot cocoa and, in the general way of things this is not incorrect. The fine shade of meaning between made of chocolate and tasting of chocolate being rather beyond most people's interest.
Thus, there are many brands of powdered mixes on the market made with cocoa yet called hot chocolate. Though, as we all know, for absolute clarity, such beverages should be called hot cocoa. In some cases, the mixes are quite good and meant to be used with hot milk. In some cases, the mixes are quite bad and are intended to be acceptable when made with water. A good one is Ghirardelli Sweet Ground Chocolate and cocoa. The name for this product predates our bickering over its usage, so it should perhaps be forgiven.1
A great irony, of course, is that hot chocolate was originally made with water. Just as it was not sweet. Of course, at the time, it was made with ground chocolate, not cocoa. Having used water with cocoa I can assure you that, while the flavor is more robust than when made with milk, cocoa does not remain in suspension in water as long as it does in milk. Thus, hot cocoa is made with milk. This problem does not exist with hot chocolate made with water, however, and I greatly urge you to try it this way. This is not some aberration and hot chocolate is still made with water in many parts of the world. The water allows the chocolate's flavor to bloom more fully.
Of late years, there has been an upsurge of high-end chocolate drinking. True hot chocolate made from chocolate, cocoa butter intact, has become more common and more readily available prepackaged. The irony to this is that, since chocolate is readily available, prepackaged chocolate specifically for hot chocolate is somewhat unnecessary.
Although it is simple enough to make hot chocolate at home without a mix, there are several on the market. There is the classic Mexican hot chocolate, available for many years from Ibarra amongst others. Formed into tablets, granular and often with a hint of cinnamon, a chunk is added to hot water or milk, melted and stirred in. A pleasant froth can be made by using a whisk, immersion blender, or standard blender to do the mixing.
Several companies have come out more recently with ground mixes. They are high end, luxury chocolatiers such as Marie Belle (a shop in Soho, New York with a web storefront as well), Chuao Chocolatier, and Lake Champlain Chocolates. These newer products are sweetened chocolate, ground to mix more quickly into hot milk or water. Indeed, Marie Belle encourages everyone to try their hot chocolate made with water and adds that it is "European style" to do so. Marie Belle also explains that their hot chocolate can be left to cool and as such turns into a rich pudding. I have tried this, and it is indeed true. Largely because their European style hot chocolate is half chocolate; essentially ganache made with water. Somehow, I still prefer regular chocolate pudding, however.
The newer products also tend to play up the Aztec use of chocolate, offering blends which include chili powder, and often romanticizing its history. Determining whether this is interesting or mawkish is a personal exercise, I think.
In the end, all of these mixes are essentially chocolate and sugar mixed into pleasing proportions and intended to be melted into hot liquid and then drunk. It's easy enough to do the same, at home, with sweetened bar chocolate or unsweetened chocolate and sugar, or both. I recommend using a high quality chocolate as it will make a difference. There are several nodes with more information on making hot chocolate from 'scratch,' including Mexican hot chocolate, drinking chocolate, hot chocolate, Toblerone-based Hot Chocolate with Marshmallows filled with Mint Pesto, and Hot chocolate to die for.
1 Information on Ghirardelli's Sweet Ground Chocolate and Cocoa comes from The Global Gourmet which gleaned it from the Ten Speed Press, publishers of the Ghirardelli Chocolate Cookbook.