While you may see the word "espresso" marked on a bag of beans, it's not actually a type of roast or a particular coffee bean; it's simply a method of preparing coffee.
There are a few differences between making espresso and brewing coffee with something like a French press or Chemex. For starters, an espresso machine is a completely different contraption, and because of the extraction method — which involves hot water at a high pressure — beans are ground much finer than what you would use in a pour-over method. This method makes for a smaller, more intense drink — something that many coffee drinkers prefer.
The extraction method, because of the pressure and the temperature of the water, also takes a much shorter amount of time than when you manually brew coffee.
What's the Deal with Espresso Crema?
Besides its small size and concentrated flavor, one of the main indicators of an espresso is the crema. This is the layer of foam on top of the espresso shot, similar to what you might get in a pint of beer. I recently learned about the science of crema.
You can learn a couple things about an espresso from the crema. First: whether or not the beans used to make the espresso were fresh. When water is under high pressure (like it is in an espresso extraction), it is able to dissolve more carbon dioxide, which is produced in coffee during the roasting process. When the extracted coffee hits the cup, and the liquid comes back to a normal pressure, the liquid can't hold onto all of that gas, so it comes to the top in the form of small bubbles — which chemicals in the coffee attach to, in turn creating the crema. So if a coffee is freshly roasted, it will have more crema, because more foam will be produced. Don't see a good-looking crema? That's a sign that the coffee used was older.
The crema is also an indicator of the strength of the coffee; the darker the crema, the stronger the espresso is.