PUMP vs. STEAM (No, we're not talking about boats here). There are two types of espresso machines to choose from and the difference is substantial.
PUMP DRIVEN is the higher-end type of espresso machine. Water is heated to the right temperature and then forced through the grinds using an internal pump. This force is more intense than in a steam machine and results in a higher quality and better tasting cup of espresso. Pump driven machines consistently produce the high BARS OF PRESSURE necessary for full-flavored espresso. The water tank can be refilled at any time for unlimited amounts of espresso, as opposed to the steam driven machine, which must cool down between brewing. Pump machines do not have a carafe, therefore you place your cup directly under the spout. Only a pump driven machine produces CREMA.
NOTE: CREMA is a golden foam-like layer at the top of a freshly-brewed shot of espresso. It makes a "cap" over the espresso liquid, which helps to retain the flavors and aromas of the espresso. CREMA is the hallmark of a well-prepared espresso and can only be achieved through the use of a pump-driven machine. Remember - CREMA is not to be confused with cream or frothed milk that tops specialty drinks. We'll get to that in a moment. The BARS OF PRESSURE is the measurement of steam pressure which is being pushed through the grinds. Higher bars of pressure are necessary to produce full-flavor espresso. Between 15 and 18 bars of pressure are typically at the higher end of personal espresso machines.
Pump driven machines can be further classified as Super Automatic, Fully Automatic and Semi-Automatic.
Super Automatic machines are fully automatic machines with additional features for the ultimate espresso experience. These features may include larger reservoirs, separate heating chambers for steaming and brewing and automatic controls and indicators that monitor the various processes.
Full Automatic machines precisely grind, measure, tamp, brew and dispose of the used grids all at the touch of a button. The have specialized brewing processes that optimize flavor extraction.
Semi-Automatic machines require the user to manually control the length/volume of the espresso by using a dial or lever to start and stop the brewing process.
Some of these pump-driven machines have many features such as adjustable coffee strength options, multiple cup sizes for those double espresso drinkers and automatic frothers - all features that add to the quality and price of the machine. Additional features on certain models are a removable drip tray, warming tray, and greater water capacity. You can also purchase other products such as a coffee grinder, frothing pitcher and demitasse/cappuccino cups to add to your espresso enjoyment.
STEAM DRIVEN is the more moderately priced type of espresso machine. Steam-driven espresso machines work by boiling water and forcing it through espresso grinds by means of steam pressure. Because this type of machine relies solely on steam pressure to push water through a bed of grinds, the espresso is generally not as full-flavored as the espresso produced with a pump machine. A steam-driven machine can only brew one carafe (approximately four shots of espresso) at a time. It can not be opened during the process to add more water due to the boiling hot water that is necessary to operate it; therefore there is a lag time between brewing cycles. The machine must cool completely before it is possible to add more water and brew additional servings.