You can thank the Spanish conquistadors for bringing chocolate to Europe in the 16th century from the Aztecs, who learned about chocolate from the Mayans. However, today's "Mexican chocolate" is entirely different from common milk chocolate -- it's a variant on the European version of chocolate that was a variant on the original chocolate consumed by the Aztecs, in what is now Mexico. Got all that?
Present-day Mexican chocolate is savory, sweet, and made up of several different flavors, not just chocolate and sweetness. It includes dark, bitter raw chocolate, with added sugar, cinnamon, and usually ground nuts. Unlike other chocolates, such as milk and dark chocolate, which are at their best if they are smooth, Mexican chocolate is at its best when it's grainy. It's sold in disks so it can be consumed like a candy bar. As for the Aztec version of chocolate, it was similar to today's Mexican chocolate, and even more similar to a coffee shop's Mexican mocha drink: It was a hot beverage made of raw chocolate, spices, seeds and honey (which was readily available 400 years ago).