There are two ways to make espresso, steam and pump. The steam models are the stovetop boilers and many countertop pressure units. The stovetop boilers, called Moka pots are fairly inexpensive, small, two part espresso pots. Widely used in Europe, they are considered inferior by many espresso connoisseurs today, because they produce a pretty bitter espresso. The higher temperature of the steam-driven, boiling-hot water that is forced through the grounds is the culprit. There is no crema, and the resulting product can sometimes have an almost burnt flavor. The countertop pressure machines are fairly lightweight and have a steam wand for frothing milk. These units produce an acceptable cup of espresso and are best for those folks who want espresso on an occasional to semi-regular basis.
Pump units are the types of machines used in espresso bars, and are the best investment for the true espresso addict. The water is kept at slightly below boiling temperature (approximately 192°F), with a buildup of steam pressure measured in atmospheres. The steam is then forced through the coffee grounds by hand or machine pumping. This produces the most consistent and flavorful espresso.
Espresso machines can range in cost from as little as $50 for a small Moka pot into the thousands of dollars for a top of the line pump model.