Source: http://www.espressoequipment.org/265/how-an-espresso-machine-works.html/

Espresso is a drink made by forcing water heated to just below boiling point through a filter packed, finely ground coffee beans.
A single serving of espresso is a 1. 25-1. 5 oz coffee concentrate covered with a thin layer of foam (called Crema). Known for its rich body and aroma, the bitter taste of espresso stops on the palette after each sip, the taste of satisfaction for coffee lovers.
While it is possible to construct a crude method of brewing, forcing hot water into the coffee using steam power, specialized machinery needed to prepare the perfect shot of espresso.
An espresso machine heats the water to the recommended temperature controls water pressure and may even provide an attachment of the steam nozzle to heat the milk and foam is added to the drink. The typical home espresso machine has a tank where cold water is poured. There is also a removable handle attached to a filter (called portafilter), which is just above the platform where you place the cup of espresso.
When the machine is on, the water tank is pumped through a heating element. Once this element heats the water to the desired temperature (about 190 degrees Fahrenheit), an indicator light informs the user of the process of brewing is ready to begin.
Before mixing, the user fills out the portafilter with ground coffee and tamping the grounds. The portafilter is placed in the machine and locked into place. A switch is overruled by the brewery that signals the pump to the machine to pressurize the chamber is heated to about 220 psi, forcing the water heater through the ground coffee, with a peak at the bottom the portafilter.
Once the glass is full, the espresso machine is switched off and a cup of coffee is ready to be enjoyed.

Although most espresso machines use this basic process for preparing an espresso, prices can vary from fifty dollars to thousands. Additional features of more expensive units have the capacity to moisten the soil before preparation begins, the pumps can produce higher quality more sustained pressure, the increase in heating systems, integrated water filter foam sticks.
Achieving a good cup of coffee from a machine at home does not cost thousands of dollars, however. Simple procedures can add the taste of coffee, even in low-end machines. Fill the tank with filtered water chemical reactions take place in the process of preparation. The type of coffee bean and the fineness of the grind has a direct impact on the overall flavor and should be tested to find the perfect mix.

Furthermore, of the compactness of the reasons for the portafilter affects the emulsification process, whose results are visible in the foam. Perhaps the most important quality of a good espresso is the freshness. Using the new grounds take full coffee flavor comes through. Coffee should be consumed within a few minutes of experience in brewing flavor. With a little experimentation, the same as ordered in a coffee can be enjoyed at home with a home brewer of espresso. The satisfaction of having perfect espresso infused, however, never be in a menu of Starbuck.