The operation of the espresso machine we find in coffee shops or at home is quite similar from a procedural point of view, what varies are the dimensions (especially of the internal operating elements) which are then reflected in the final quality of the espresso coffee dispensed.
Conceptually, an espresso machine is composed of a boiler containing boiling water at about 1.2 bar; the moment we press the button here we call up a portion of water from the boiler and, after a few seconds of pre-infusion, the electric pump forces the water through the ground coffee pod, extracting the substances and aromas of the coffee in a very energetic way.
The espresso machine therefore always consists of the following indispensable elements:
Boiler: this is generally made of steel, copper or brass. Its purpose is to heat and keep the water hot through a heat exchanger.
Electric pump: this is the instrument capable of crushing water against the coffee pod with a pressure around 9 atmospheres. A pressure gauge on the machine should indicate this extraction pressure.
Group: is the arm that is gripped to put in and take out the coffee doses. In addition to the knob, it consists of a portafilter container that houses the ground and pressed coffee.
The difference between a home machine and a bar espresso machine lies in size:a smaller boiler, found in home machines, has more difficulty keeping the water hot enough. A "heavy" machine structure, thus with a lot of metal, will also allow the pressure and thus the heat of the water to be preserved best.
English
Italian 
German 
Spanish