Smithsonian has a wonderful article on the history of espresso but this concluding paragraph summarizes what espresso is:
Over more than a century, the espresso machine has been drastically improved, with electrical components, computerized measurements, and portable pneumatics. But as with the finest objects of design, science and technology is not enough. There is an art to the espresso as well. The talent of the barista is as important as the quality of the beans and the efficiency of the machine. Indeed, it is said that a good espresso depends on the four M’s: Macchina, the espresso machine; Macinazione, the proper grinding of a beans –a uniform grind between fine and powdery– which is ideally done moments brewing the drink;Miscela, the coffee blend and the roast, and Mano is the skilled hand of the barista, because even with the finest beans and the most advanced equipment, the shot depends on the touch and style of the barista. When combined properly, these four Ms yield a drink that is at once bold and elegant, with a light, sweet foam crema floating over the coffee. A complex drink with a complex history.