Hi there, great question!
So mocha is a type of coffee drink. Since coffee and chocolate are some of the most popular combinations and definitely one of the most delicious ones as well, mocha is the brainchild of a genius who thought of this fantastic blend!
It would also interest you to know that the term "mocha" comes from Al Mokha, a port city on Yemen's southwestern coast. This place was actually one of the world's first international hubs for coffee exportation. Yemen's dark coffee beans are known to be aromatic and have the rich taste of cacao. And this is actually where the association of coffee and chocolate began.
This combo is actually a blend of cappuccino and hot chocolate. A cappuccino contains espresso, warm milk, and a frothy top, a mocha contains some delicious, sweet, chocolate in the form of syrup or powder. It's very similar to a latte, except the latte only has milk and whereas mocha has chocolate.
There also exists something known as a "white mocha," which is actually pretty similar to a regular mocha but has white chocolate rather than dark chocolate (pretty self-explanatory, right?).
Mocha is a type of coffee drink; hence it contains espresso, but there are also decaffeinated options available. One can add different variations of chocolate in their mocha. Some also use hazelnut or milk chocolates to spice things up a little.
What also makes the perfectly creamy cup of mocha is actually steamed and frothed milk. This is the type of milk that will leave a chocolatey mustache on your lip and warms up your soul. Full cream milk would be ideal, but a mocha can also have low-fat or dairy-free alternatives.
You can actually try and make your own iced or hot mocha, all you need is coffee milk chocolate powder.
I hope this answers your question!