What is espresso?

Espresso is a method of brewing coffee, otherwise known as an extraction method. It is not a specific coffee bean or roast type! The major difference between espresso and other coffee brewing methods is pressure. To brew espresso, hot water is forced through a bed of finely ground coffee at high pressure, typically between 6 and 9 BAR. The contact between the coffee and the water at pressure results in a higher level of extraction, meaning that the natural oils and gasses in the coffee grounds are drawn out. The result is a stronger, more robust, but smaller coffee beverage with a layer of natural foam on top, called crema.
While espresso is higher in caffeine, per ounce, than regular brewed coffee, one or two shots of espresso normally will have less caffeine overall than an 8oz cup of brewed coffee.