What is Espresso Coffee (Expresso)

Espresso shot with crema

Espresso coffee is coffee made using a method of "extracting" the coffee under pressure with very hot water, though not boiling, in an espresso coffee machine. 

The method was invented in Italy at the beginning of the 20th century, and at first involved using steam to extract the coffee.  In the 1940's, the "spring piston lever machine" was invented, and is the precursor of the modern espresso machine and method of making espresso coffee.

"Expresso" is the French variation of the Italian word, and is a misspelling and mispronunciation not-uncommon in English which has adopted the original Italian word, "espresso".

The coffee beans used for making espresso coffee are chosen for their ability to produce "crema", and they are ground finely, between the powdery grind used for Turkish coffee, and the medium grind used for drip-filter systems.

When the coffee is extracted under the right temperature and pressure, the coffee oils, sugars and proteins are caramelized creating what is known as "crema", and essential element in good espresso coffee.
 
Crema floats on the top of the extracted espresso coffee and as has a color that can range from light-cream which can in fact be mistaken for milk when mid- to light-roast beans have been used, to reddish-brown, when dark-roast beans have been used. I like to think of crema is the "liqueur" of espresso coffee, imparting the unique flavor of espresso coffee, and without which the resulting coffee is typically bitter and unpleasant to drink, and not worth drinking.