Espresso is a particular way of preparing coffee. In other words, all espresso is coffee, but not all coffee is espresso. Coffee is a beverage made from the roasted seeds of the Coffea plant, or "coffee beans," but there are many ways of making the beans into a beverage.
In the United States, when one hears the word "coffee," or sees it on a restaurant menu, it is usually safe to assume that the beverage is made using the drip or percolator method. "Espresso" is typically referred to by that name in order to differentiate it from other types of coffee. In other countries, however, espresso is sometimes referred to simply as "coffee," while what those in the United States most commonly drink may be called "American coffee" or may not even be available.
Espresso is made by forcing a shot of hot water at high pressure through very finely ground coffee, while electric percolators and drip machines use gravity to move the water through the grounds. The high pressure method results in a brew that is more concentrated, thicker, darker, and stronger in flavor than gravity-brewed coffee, though the latter has a higher concentration of caffeine. In addition to being served on its own, espresso is used in a number of specialty coffee drinks, including lattes and mochas. It is well suited to these drinks because it is so concentrated that a relatively small amount can flavor an entire beverage.
In the United States, coffee is sometimes sold as "espresso roast," typically a dark roast with an oily surface. However, any type of coffee bean at any roast level may be used to make espresso. Many people prefer lighter roasts, and the types of beans used are largely a matter of personal taste. Conversely, espresso roast beans do not have to be used to make espresso; they are just as suitable for drip coffee and other brewing methods.
It is important, however, to use only "espresso grind" coffee in an espresso maker, as other grinds are too coarse. This grind is likewise too fine for many other coffee-making methods. Using a very fine grind in a drip coffeemaker, for example, will produce a bitter drink full of grounds.