Espresso is a way of making coffee under pressure, and not a bean type, bean origin, or any special kind of coffee bean, despite what anyone will tell you.
At some point or other, we've all found ourselves in front of a bag of "chocolate covered espresso beans". Or about to taste Sambuca, and given espresso beans to plop in. Truth is, there's just regular coffee beans. Nothing different whatsoever.... Somehow the myth that there is a type of bean specifically meant for espresso is everywhere!
Espresso is a way of preparing coffee, pure and simple, that uses hot water under pressure to extract coffee from the ground beans. The name espresso literally means "under pressure", and not "super fast" (the other misconception: that it's eXpresso). Generally, the beans used to make espresso are comprised of medium to dark roasted beans, and are blends specifically formulated to produce the best part of an espresso shot: the thick, delicious crema.
Another common misconception is that just as shots of espresso taste super strong (concentrated flavor), they're also higher in kick. In fact, even a generous double shot of espresso can have about a third of the caffeine you'd find in a mug of drip coffee.
If you order your shot without milk, something we highly recommend if you haven't yet (most espresso shots in the US are explicitly made with milk, like in the case of lattes, cappuccinos, etc.), you can tell very quickly whether you have a good shot when it's covered in a layer of crema, which will generally have some tiny bubbling or dark spotting as well. This is the thick, creamy layer of foam that balances on top of the shot. It's an artifact of the extraction of the coffee under high pressure (about 130 pounds per square inch!).
Espresso represents a perfectly caffeinated sip that is truly an art and a science, and the mythical beast of the "espresso bean" should forever remain in the realm of confectioners and Italian restaurants.