The main differences between espresso coffee and drip coffee are the fineness of the grind and the brewing time. The brewing time for espresso coffee is much shorter, made possible by espresso machines that generate up to 15 atmospheres (ATM) of pressure to force the water through the coffee.
A shot of espresso is made by forcing about 1.5 ounces of nearly boiling water through tightly packed, finely ground espresso coffee. If everything goes well, what comes out is a dark brown, slightly thick liquid with a small amount of crema (a foam, sort of like the head on a beer) on top.
There are many variables in the process of making a shot of espresso. The temperature of the water, the pressure of the water, the fineness of the ground coffee and how tightly the coffee is packed are just a few.
Espresso coffee is a blend of several different types of coffee beans from different countries. The beans are roasted until they are dark and oily-looking.
Coffee beans (left) and ground espresso coffee (right)
The beans are ground very finely -- much finer than for drip coffee. The consistency is almost like powdered sugar. The more finely the coffee is ground, the slower the espresso comes out. Generally, for the best shot of espresso, it should take about 25 seconds for the water to pass through the coffee. Sometimes, the consistency of the grind is adjusted to control the brewing time.
Drip coffee is made by dripping boiling water over ground coffee, which is ground more coarsely than espresso coffee. The water filters through the coffee and falls into a pot. This process is slower than the espresso process, and hot water is in contact with the ground coffee for much longer. Surprisingly, a cup of drip coffee has more caffeine than a shot of espresso. 
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