Espresso is an Italian style of coffee that's brewed very strong and served in a small cup. When you add steamed milk to espresso, you get a latte or a cappuccino.
Espresso is different from regular American coffee in many ways. It's typically made using dark roasted coffee beans that are very finely ground and brewed in a special espresso machine that forces hot water at high pressure through the ground coffee. The resulting espresso is dark, ideally with a layer of creamy-colored "crema" on the top. In Italian, espresso means "pressed out," from the Latin exprimere, "squeeze out."