Most coffee experts consider brewing coffee quickly and under high pressure the best process to extract maximum flavor and aroma from your coffee. High pressure and the shortened brewing time help to prevent extraction of some of the bitter oils found in coffee and also help to release less caffeine than traditional brewing methods.
When you use the technique of high pressure and a quick brewing cycle to make small amounts of coffee it is considered an espresso. The pressure is the result of the packed or tamped coffee giving the water resistance to pass through. The espresso machine pump forces the water through the packed ground coffee. When espresso is made properly, including the correct grind, tamping and the pressure brewing your espresso will boast a foamy layer on top, called "crema". What the crema does is trap the fine aromas and the light flavors that make espresso so delicious. The crema keeps the flavors, which would other wise escape, locked in. Crema is the trademark of a well prepared espresso