Contrary to what you might think, espresso is not a roasting method, or bean or blend. Rather, it's a method of preparation so precise and molecular that no discussion of espresso could exist without discussing the machines and how they changed the whole business.
In the 19th century, coffee was at the height of its popularity in Europe, but the brewing process was slow. Various inventors thus began exploring ways of using steam to reduce the brewing time, but it was Angelo Moriondo of Turin, Italy and his 1884 patent for a "new steam machinery for the economic and instantaneous confection of coffee beverage" that would eventually lead to espresso and the espresso machine.
Unfortunately, Moriondo's machine was lost to history, but Luigi Bezzerra and Desiderio Pavoni (considered now as "the Steve Wozniak and Steve Jobs of espresso") took Moriondo's design and improved it, eventually developing the first single-shot espresso, which only took a few seconds to brew. While Bezzera's first machine was heated over an open flame, making it difficult to control pressure and temperature, it was Pavoni who invented the pressure release valve, and the new machine premiered at the 1906 Milan Fair.
These early machines could produce up to 1,000 cups of coffee per hour, but relied exclusively on steam, which had the unfortunate side effect of imbuing the coffee with a burnt or bitter taste and could only conjure up, at best, two bars of atmospheric pressure - not even enough for the resulting drink to be considered espresso by today's standard. As electricity replaced gas and Art Deco replaced the chrome-and-brass aesthetic of the early 20th century, the machines became smaller and more efficient, but no coffee innovators managed to create a machine that could brew with more than 1.5-2 bars of pressure without burning the coffee.
It wasn't until after World War II that the first machine to surpass the two-bar brewing barrier was invented. Milanese café owner Achille Gaggia—credited with the "birth of modern espresso"—increased the water pressure from 1.5-2 bars to 8-10 bars through the use of a lever, and standardized the size of the espresso. The next big revolution in espresso came in the 1960's, with the invention of the motorized pump. The E61 machine by Ernesto Valentean "was an immediate success and is rightly included in the pantheon of the most influential coffee machines of history."
It's a great read if you love nitty-gritty historical details!
Read More: The Long History of the Espresso Machine at Smithsonian Magazine
Related: The Art of Making a Cappuccino
(Image: Espresso Machine Classics via Smithsonian Magazine)